CLEANUP: makefile: properly indent ifeq/ifneq conditional blocks

Some conditional blocks have become out of control over time and are
totally unreadble. It took 15 minutes to figure what "endif" matched
what "if" in the PCRE one for example, and DA and 51D use multiple
levels as well that are not easy to sort out.

Let's reindent the whole thing. Most places that were already indented
used 2 spaces per level, so here we're keeping that principle. It was
just not done on the two last ones that are used to define some rules
because we don't want spaces before rule names. A few had the opening
condition indicated on the endif line.

It would be desirable that over time this more maintainable layout is
preserved.

Makefile

@@ -750,7 +750,7 @@ else
     OPTIONS_CFLAGS  += $(if $(PCRE_INC),-I$(PCRE_INC))
     OPTIONS_LDFLAGS += $(if $(PCRE_LIB),-L$(PCRE_LIB)) -Wl,-Bstatic -lpcreposix -lpcre -Wl,-Bdynamic
   endif
-endif
+endif # USE_PCRE
 
 ifneq ($(USE_PCRE2)$(USE_STATIC_PCRE2)$(USE_PCRE2_JIT),)
   # in case only USE_STATIC_PCRE2/USE_PCRE2_JIT were set
@@ -773,7 +773,6 @@ endif
       endif
     endif
 
-
     PCRE2_LDFLAGS := $(shell $(PCRE2_CONFIG) --libs$(PCRE2_WIDTH) 2>/dev/null || echo -L/usr/local/lib -lpcre2-$(PCRE2_WIDTH))
 
     ifeq ($(PCRE2_LDFLAGS),)
@@ -792,9 +791,8 @@ PCRE2_LDFLAGS   := $(if $(PCRE2_LIB),-L$(PCRE2_LIB)) -Wl,-Bstatic -L$(PCRE2_LIB)
     else
       PCRE2_LDFLAGS := $(if $(PCRE2_LIB),-L$(PCRE2_LIB)) -L$(PCRE2_LIB) $(PCRE2_LDFLAGS)
     endif
-
-endif
-endif
+  endif # PCRE2DIR
+endif # USE_PCRE2
 
 ifneq ($(USE_NS),)
   OPTIONS_OBJS  += src/namespace.o
@@ -893,8 +891,8 @@ all:
 	@exit 1
 else
 all: haproxy dev/flags/flags $(EXTRA)
-endif
-endif
+endif # obsolete targets
+endif # TARGET
 
 OBJS =
 
@@ -970,7 +968,7 @@ build_opts = $(shell rm -f .build_opts.new; echo \'$(TARGET) $(BUILD_OPTIONS) $(
 .build_opts: $(build_opts)
 else
 .build_opts:
-endif
+endif # reg-tests
 
 haproxy: $(OPTIONS_OBJS) $(OBJS)
 	$(cmd_LD) $(LDFLAGS) -o $@ $^ $(LDOPTS)