-D_FORTIFY_SOURCE=2 is enable in gcc by default on alpine. See the
0004-Turn-on-D_FORTIFY_SOURCE-2-by-default-for-C-C-ObjC-O.patch patch in
main/gcc. Disabling it here silences a bunch of compiler warnings.

--- a/Makefile.in
+++ b/Makefile.in
@@ -39,7 +39,7 @@ DEFS = @DEFS@ -DNMAP_PLATFORM=\"$(NMAP_P
 # http://gcc.gnu.org/ml/gcc-patches/2004-09/msg02055.html
 # Level 1 only makes changes that don't affect "conforming" programs,
 # while level 2 enforces additional restrictions.
-DEFS += -D_FORTIFY_SOURCE=2
+# DEFS += -D_FORTIFY_SOURCE=2
 # For mtrace debugging -- see MTRACE define in main.cc for instructions
 # Should only be enabled during debugging and not in any real release.
 # DEFS += -DMTRACE=1