-D_FORTIFY_SOURCE=2 is enable in gcc by default on alpine. See the
010_all_default-fortify-source.patch patch in main/gcc. Disabling
it here silences a bunch of compiler warnings.

diff -upr nmap-7.10.orig/Makefile.in nmap-7.10/Makefile.in
--- nmap-7.10.orig/Makefile.in	2016-03-20 00:47:16.555111217 +0100
+++ nmap-7.10/Makefile.in	2016-03-20 00:47:26.348392650 +0100
@@ -41,7 +41,7 @@ DEFS = @DEFS@ -DNMAP_NAME=\"$(NMAP_NAME)
 # 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