diff --git a/INSTALL b/INSTALL
index ad0596b87..1268f2351 100644
--- a/INSTALL
+++ b/INSTALL
@@ -73,10 +73,10 @@ can use a relatively similar one and adjust specific variables by hand.
 Most configuration variables are in fact booleans. Some options are detected and
 enabled by default if available on the target platform. This is the case for all
 those named "USE_<feature>". These booleans are enabled by "USE_<feature>=1"
-and are disabled by "USE_<feature>=" (with no value). An exhaustive list of the
-supported USE_* features is located at the top of the main Makefile. The last
-occurrence of such an option on the command line overrides any previous one.
-Example :
+and are disabled by "USE_<feature>=" (with no value) or "USE_<feature>=0". An
+exhaustive list of the supported USE_* features is located at the top of the
+main Makefile. The last occurrence of such an option on the command line
+overrides any previous one. Example :
 
   $ make TARGET=generic USE_THREAD=
 
@@ -749,9 +749,10 @@ multithreading via USE_THREAD.
 You can easily define your own target with the GNU Makefile. Unknown targets
 are processed with no default option except USE_POLL=default. So you can very
 well use that property to define your own set of options. USE_POLL and USE_SLZ
-can even be disabled by setting them to an empty string. For example :
+can even be disabled by setting them to an empty string or a zero. For
+example :
 
-  $ gmake TARGET=tiny USE_POLL="" USE_SLZ="" TARGET_CFLAGS=-fomit-frame-pointer
+  $ gmake TARGET=tiny USE_POLL="" USE_SLZ=0 TARGET_CFLAGS=-fomit-frame-pointer
 
 If you need to pass some defines to the preprocessor or compiler, you may pass
 them all in the DEFINE variable. Example:
diff --git a/Makefile b/Makefile
index 71a8c2179..76b6cfa6b 100644
--- a/Makefile
+++ b/Makefile
@@ -333,7 +333,7 @@ USE_POLL   = default
 
 # SLZ is always supported unless explicitly disabled by passing USE_SLZ=""
 # or disabled by enabling ZLIB using USE_ZLIB=1
-ifeq ($(USE_ZLIB),)
+ifeq ($(USE_ZLIB:0=),)
   USE_SLZ    = default
 endif
 
@@ -470,7 +470,7 @@ $(set_target_defaults)
 # any occurrence of 1 indicates libatomic is necessary. It's better to avoid
 # linking with it by default as it's not always available nor deployed
 # (especially on archs which do not need it).
-ifneq ($(USE_THREAD),)
+ifneq ($(USE_THREAD:0=),)
   ifneq ($(shell $(CC) $(CFLAGS) -dM -E -xc - </dev/null 2>/dev/null | grep -c 'LOCK_FREE.*1'),0)
     USE_LIBATOMIC   = implicit
   endif
@@ -522,7 +522,7 @@ BUILD_OPTIONS  := $(call build_options)
 # possibly be unused though)
 OPTIONS_CFLAGS += $(call opts_as_defines)
 
-ifneq ($(USE_LIBCRYPT),)
+ifneq ($(USE_LIBCRYPT:0=),)
   ifneq ($(TARGET),openbsd)
     ifneq ($(TARGET),osx)
       LIBCRYPT_LDFLAGS = -lcrypt
@@ -530,45 +530,45 @@ ifneq ($(USE_LIBCRYPT),)
   endif
 endif
 
-ifneq ($(USE_ZLIB),)
+ifneq ($(USE_ZLIB:0=),)
   # Use ZLIB_INC and ZLIB_LIB to force path to zlib.h and libz.{a,so} if needed.
   ZLIB_CFLAGS      = $(if $(ZLIB_INC),-I$(ZLIB_INC))
   ZLIB_LDFLAGS     = $(if $(ZLIB_LIB),-L$(ZLIB_LIB)) -lz
 endif
 
-ifneq ($(USE_SLZ),)
+ifneq ($(USE_SLZ:0=),)
   OPTIONS_OBJS   += src/slz.o
 endif
 
-ifneq ($(USE_POLL),)
+ifneq ($(USE_POLL:0=),)
   OPTIONS_OBJS   += src/ev_poll.o
 endif
 
-ifneq ($(USE_EPOLL),)
+ifneq ($(USE_EPOLL:0=),)
   OPTIONS_OBJS   += src/ev_epoll.o
 endif
 
-ifneq ($(USE_KQUEUE),)
+ifneq ($(USE_KQUEUE:0=),)
   OPTIONS_OBJS   += src/ev_kqueue.o
 endif
 
-ifneq ($(USE_EVPORTS),)
+ifneq ($(USE_EVPORTS:0=),)
   OPTIONS_OBJS   += src/ev_evports.o
 endif
 
-ifneq ($(USE_RT),)
+ifneq ($(USE_RT:0=),)
   RT_LDFLAGS = -lrt
 endif
 
-ifneq ($(USE_THREAD),)
+ifneq ($(USE_THREAD:0=),)
   THREAD_LDFLAGS = -pthread
 endif
 
-ifneq ($(USE_BACKTRACE),)
+ifneq ($(USE_BACKTRACE:0=),)
   BACKTRACE_LDFLAGS = -Wl,$(if $(EXPORT_SYMBOL),$(EXPORT_SYMBOL),--export-dynamic)
 endif
 
-ifneq ($(USE_CPU_AFFINITY),)
+ifneq ($(USE_CPU_AFFINITY:0=),)
   OPTIONS_OBJS   += src/cpuset.o
 endif
 
@@ -580,32 +580,32 @@ endif
 
 # This is for the WolfSSL variant of the OpenSSL API. Setting it implies
 # OPENSSL so it's not necessary to set the latter.
-ifneq ($(USE_OPENSSL_WOLFSSL),)
+ifneq ($(USE_OPENSSL_WOLFSSL:0=),)
   SSL_CFLAGS      := $(if $(SSL_INC),-I$(SSL_INC)/wolfssl -I$(SSL_INC))
   SSL_LDFLAGS     := $(if $(SSL_LIB),-L$(SSL_LIB)) -lwolfssl
   # always automatically set USE_OPENSSL
-  USE_OPENSSL     := $(if $(USE_OPENSSL),$(USE_OPENSSL),implicit)
+  USE_OPENSSL     := $(if $(USE_OPENSSL:0=),$(USE_OPENSSL:0=),implicit)
 endif
 
 # This is for the AWS-LC variant of the OpenSSL API. Setting it implies
 # OPENSSL so it's not necessary to set the latter.
-ifneq ($(USE_OPENSSL_AWSLC),)
+ifneq ($(USE_OPENSSL_AWSLC:0=),)
   # always automatically set USE_OPENSSL
-  USE_OPENSSL     := $(if $(USE_OPENSSL),$(USE_OPENSSL),implicit)
+  USE_OPENSSL     := $(if $(USE_OPENSSL:0=),$(USE_OPENSSL:0=),implicit)
 endif
 
 # This is for any variant of the OpenSSL API. By default it uses OpenSSL.
-ifneq ($(USE_OPENSSL),)
+ifneq ($(USE_OPENSSL:0=),)
   # only preset these for the regular openssl
-  ifeq ($(USE_OPENSSL_WOLFSSL),)
+  ifeq ($(USE_OPENSSL_WOLFSSL:0=),)
     SSL_CFLAGS    := $(if $(SSL_INC),-I$(SSL_INC))
     SSL_LDFLAGS   := $(if $(SSL_LIB),-L$(SSL_LIB)) -lssl -lcrypto
   endif
-  USE_SSL         := $(if $(USE_SSL),$(USE_SSL),implicit)
+  USE_SSL         := $(if $(USE_SSL:0=),$(USE_SSL:0=),implicit)
   OPTIONS_OBJS += src/ssl_sock.o src/ssl_ckch.o src/ssl_sample.o src/ssl_crtlist.o src/cfgparse-ssl.o src/ssl_utils.o src/jwt.o src/ssl_ocsp.o src/ssl_gencert.o
 endif
 
-ifneq ($(USE_ENGINE),)
+ifneq ($(USE_ENGINE:0=),)
   # OpenSSL 3.0 emits loud deprecation warnings by default when building with
   # engine support, and this option is made to silence them. Better use it
   # only when absolutely necessary, until there's a viable alternative to the
@@ -613,7 +613,7 @@ ifneq ($(USE_ENGINE),)
   ENGINE_CFLAGS   = -DOPENSSL_SUPPRESS_DEPRECATED
 endif
 
-ifneq ($(USE_QUIC),)
+ifneq ($(USE_QUIC:0=),)
 OPTIONS_OBJS += src/quic_conn.o src/mux_quic.o src/h3.o src/xprt_quic.o    \
                 src/quic_frame.o src/quic_tls.o src/quic_tp.o              \
                 src/quic_stats.o src/quic_sock.o src/proto_quic.o          \
@@ -627,11 +627,11 @@ OPTIONS_OBJS += src/quic_conn.o src/mux_quic.o src/h3.o src/xprt_quic.o    \
                 src/quic_retransmit.o src/quic_fctl.o
 endif
 
-ifneq ($(USE_QUIC_OPENSSL_COMPAT),)
+ifneq ($(USE_QUIC_OPENSSL_COMPAT:0=),)
 OPTIONS_OBJS += src/quic_openssl_compat.o
 endif
 
-ifneq ($(USE_LUA),)
+ifneq ($(USE_LUA:0=),)
   check_lua_inc = $(shell if [ -d $(2)$(1) ]; then echo $(2)$(1); fi;)
   LUA_INC      := $(firstword $(foreach lib,lua5.4 lua54 lua5.3 lua53 lua,$(call check_lua_inc,$(lib),"/usr/include/")))
 
@@ -664,12 +664,12 @@ ifneq ($(USE_LUA),)
   OPTIONS_OBJS    += src/hlua.o src/hlua_fcn.o
 endif # USE_LUA
 
-ifneq ($(USE_PROMEX),)
+ifneq ($(USE_PROMEX:0=),)
   OPTIONS_OBJS    += addons/promex/service-prometheus.o
   PROMEX_CFLAGS    = -Iaddons/promex/include
 endif
 
-ifneq ($(USE_DEVICEATLAS),)
+ifneq ($(USE_DEVICEATLAS:0=),)
   # Use DEVICEATLAS_SRC and possibly DEVICEATLAS_INC and DEVICEATLAS_LIB to force path
   # to DeviceAtlas headers and libraries if needed. In this context, DEVICEATLAS_NOCACHE
   # can be used to disable the cache support if needed (this also removes the necessity of having
@@ -689,12 +689,12 @@ endif
 51DEGREES_LIB = $(51DEGREES_SRC)
 51DEGREES_VER = 3
 
-ifneq ($(USE_51DEGREES),)
+ifneq ($(USE_51DEGREES:0=),)
   ifeq ($(51DEGREES_VER),4)  # v4 here
     _51DEGREES_SRC      = $(shell find $(51DEGREES_LIB) -maxdepth 2 -name '*.c')
     OPTIONS_OBJS       += $(_51DEGREES_SRC:%.c=%.o)
     51DEGREES_CFLAGS   += -DUSE_51DEGREES_V4
-    ifeq ($(USE_THREAD),)
+    ifeq ($(USE_THREAD:0=),)
       51DEGREES_CFLAGS += -DFIFTYONEDEGREES_NO_THREADING -DFIFTYONE_DEGREES_NO_THREADING
     endif
     USE_LIBATOMIC       = implicit
@@ -703,7 +703,7 @@ ifneq ($(USE_51DEGREES),)
   ifeq ($(51DEGREES_VER),3)  # v3 here
     OPTIONS_OBJS       += $(51DEGREES_LIB)/../cityhash/city.o
     OPTIONS_OBJS       += $(51DEGREES_LIB)/51Degrees.o
-    ifeq ($(USE_THREAD),)
+    ifeq ($(USE_THREAD:0=),)
       51DEGREES_CFLAGS += -DFIFTYONEDEGREES_NO_THREADING
     else
       OPTIONS_OBJS     += $(51DEGREES_LIB)/../threading.o
@@ -720,7 +720,7 @@ ifneq ($(USE_51DEGREES),)
   USE_MATH             = implicit
 endif # USE_51DEGREES
 
-ifneq ($(USE_WURFL),)
+ifneq ($(USE_WURFL:0=),)
   # Use WURFL_SRC and possibly WURFL_INC and WURFL_LIB to force path
   # to WURFL headers and libraries if needed.
   WURFL_INC = $(WURFL_SRC)
@@ -736,12 +736,12 @@ ifneq ($(USE_WURFL),)
   WURFL_LDFLAGS    = $(if $(WURFL_LIB),-L$(WURFL_LIB)) -lwurfl
 endif
 
-ifneq ($(USE_SYSTEMD),)
+ifneq ($(USE_SYSTEMD:0=),)
   OPTIONS_OBJS    += src/systemd.o
 endif
 
-ifneq ($(USE_PCRE)$(USE_STATIC_PCRE)$(USE_PCRE_JIT),)
-  ifneq ($(USE_PCRE2)$(USE_STATIC_PCRE2)$(USE_PCRE2_JIT),)
+ifneq ($(USE_PCRE:0=)$(USE_STATIC_PCRE:0=)$(USE_PCRE_JIT:0=),)
+  ifneq ($(USE_PCRE2:0=)$(USE_STATIC_PCRE2:0=)$(USE_PCRE2_JIT:0=),)
     $(error cannot compile both PCRE and PCRE2 support)
   endif
   # PCREDIR is used to automatically construct the PCRE_INC and PCRE_LIB paths,
@@ -752,7 +752,7 @@ ifneq ($(USE_PCRE)$(USE_STATIC_PCRE)$(USE_PCRE_JIT),)
   # locations.
 
   # in case only USE_STATIC_PCRE/USE_PCRE_JIT were set
-  USE_PCRE    := $(if $(USE_PCRE),$(USE_PCRE),implicit)
+  USE_PCRE    := $(if $(USE_PCRE:0=),$(USE_PCRE:0=),implicit)
   PCRE_CONFIG := pcre-config
   PCREDIR     := $(shell $(PCRE_CONFIG) --prefix 2>/dev/null || echo /usr/local)
   ifneq ($(PCREDIR),)
@@ -761,16 +761,16 @@ ifneq ($(USE_PCRE)$(USE_STATIC_PCRE)$(USE_PCRE_JIT),)
   endif
 
   PCRE_CFLAGS := $(if $(PCRE_INC),-I$(PCRE_INC))
-  ifeq ($(USE_STATIC_PCRE),)
+  ifeq ($(USE_STATIC_PCRE:0=),)
     PCRE_LDFLAGS := $(if $(PCRE_LIB),-L$(PCRE_LIB)) -lpcreposix -lpcre
   else
     PCRE_LDFLAGS := $(if $(PCRE_LIB),-L$(PCRE_LIB)) -Wl,-Bstatic -lpcreposix -lpcre -Wl,-Bdynamic
   endif
 endif # USE_PCRE
 
-ifneq ($(USE_PCRE2)$(USE_STATIC_PCRE2)$(USE_PCRE2_JIT),)
+ifneq ($(USE_PCRE2:0=)$(USE_STATIC_PCRE2:0=)$(USE_PCRE2_JIT:0=),)
   # in case only USE_STATIC_PCRE2/USE_PCRE2_JIT were set
-  USE_PCRE2    := $(if $(USE_PCRE2),$(USE_PCRE2),implicit)
+  USE_PCRE2    := $(if $(USE_PCRE2:0=),$(USE_PCRE2:0=),implicit)
   PCRE2_CONFIG := pcre2-config
   PCRE2DIR     := $(shell $(PCRE2_CONFIG) --prefix 2>/dev/null || echo /usr/local)
   ifneq ($(PCRE2DIR),)
@@ -800,7 +800,7 @@ ifneq ($(USE_PCRE2)$(USE_STATIC_PCRE2)$(USE_PCRE2_JIT),)
       endif
     endif
 
-    ifneq ($(USE_STATIC_PCRE2),)
+    ifneq ($(USE_STATIC_PCRE2:0=),)
       PCRE2_LDFLAGS := $(if $(PCRE2_LIB),-L$(PCRE2_LIB)) -Wl,-Bstatic -L$(PCRE2_LIB) $(PCRE2_LDFLAGS) -Wl,-Bdynamic
     else
       PCRE2_LDFLAGS := $(if $(PCRE2_LIB),-L$(PCRE2_LIB)) -L$(PCRE2_LIB) $(PCRE2_LDFLAGS)
@@ -808,28 +808,28 @@ ifneq ($(USE_PCRE2)$(USE_STATIC_PCRE2)$(USE_PCRE2_JIT),)
   endif # PCRE2DIR
 endif # USE_PCRE2
 
-ifneq ($(USE_NS),)
+ifneq ($(USE_NS:0=),)
   OPTIONS_OBJS  += src/namespace.o
 endif
 
-ifneq ($(USE_LINUX_CAP),)
+ifneq ($(USE_LINUX_CAP:0=),)
   OPTIONS_OBJS   += src/linuxcap.o
 endif
 
-ifneq ($(USE_OT),)
+ifneq ($(USE_OT:0=),)
   include addons/ot/Makefile
 endif
 
 # better keep this one close to the end, as several libs above may need it
-ifneq ($(USE_DL),)
+ifneq ($(USE_DL:0=),)
   DL_LDFLAGS = -ldl
 endif
 
-ifneq ($(USE_MATH),)
+ifneq ($(USE_MATH:0=),)
   MATH_LDFLAGS = -lm
 endif
 
-ifneq ($(USE_LIBATOMIC),)
+ifneq ($(USE_LIBATOMIC:0=),)
   LIBATOMIC_LDFLAGS = -latomic
 endif
 
@@ -1157,7 +1157,7 @@ opts:
 	@#echo "$(strip $(BUILD_OPTIONS))"
 	@$(foreach opt,$(enabled_opts),\
 		$(if $(subst command line,,$(origin USE_$(opt))),,\
-			echo -n 'USE_$(opt)=$(USE_$(opt)) ';) \
+			echo -n 'USE_$(opt)=$(USE_$(opt:0=)) ';) \
 		$(if $(subst command line,,$(origin $(opt)_CFLAGS)),\
 			$(if $($(opt)_CFLAGS),echo -n '$(opt)_CFLAGS="$($(opt)_CFLAGS)" ';),\
 			echo -n '$(opt)_CFLAGS="$($(opt)_CFLAGS)" ';) \
diff --git a/include/make/options.mk b/include/make/options.mk
index d1ea90e5d..d2125866f 100644
--- a/include/make/options.mk
+++ b/include/make/options.mk
@@ -23,14 +23,15 @@ build_options   = $(foreach opt,$(use_opts),$(call ignore_implicit,$(opt)))
 # Make a list of all known features with +/- prepended depending on their
 # activation status. Must be a macro so that dynamically enabled ones are
 # evaluated with their current status.
-build_features  = $(foreach opt,$(patsubst USE_%,%,$(sort $(use_opts))),$(if $(USE_$(opt)),+$(opt),-$(opt)))
+build_features  = $(foreach opt,$(patsubst USE_%,%,$(sort $(use_opts))),$(if $(USE_$(opt):0=),+$(opt),-$(opt)))
 
-# This returns a list of -DUSE_* for all known USE_* that are set
-opts_as_defines = $(foreach opt,$(use_opts),$(if $($(opt)),-D$(opt),))
+# This returns a list of -DUSE_* for all known USE_* that are set to anything
+# neither empty nor '0'.
+opts_as_defines = $(foreach opt,$(use_opts),$(if $($(opt):0=),-D$(opt),))
 
 # Lists all enabled or disabled options without the "USE_" prefix
-enabled_opts    = $(foreach opt,$(patsubst USE_%,%,$(use_opts)),$(if $(USE_$(opt)),$(opt),))
-disabled_opts   = $(foreach opt,$(patsubst USE_%,%,$(use_opts)),$(if $(USE_$(opt)),,$(opt)))
+enabled_opts    = $(foreach opt,$(patsubst USE_%,%,$(use_opts)),$(if $(USE_$(opt):0=),$(opt),))
+disabled_opts   = $(foreach opt,$(patsubst USE_%,%,$(use_opts)),$(if $(USE_$(opt):0=),,$(opt)))
 
 # preset all XXX_{INC,LIB,CFLAGS,LDFLAGS,SRC} variables to empty for $1=XXX
 reset_opt_vars = $(foreach name,INC LIB CFLAGS LDFLAGS SRC,$(eval $(1)_$(name)=))