Define _FORTIFY_SOURCE=2 in nmap, nbase, ncat, and nsock. This

definition causes GCC to add extra security checks to compiled source
code. See http://gcc.gnu.org/ml/gcc-patches/2004-09/msg02055.html.

-D_FORTIFY_SOURCE=1 adds only those checks that are not supposed to
cause any "conforming" program to fail. (Conforming to what I'm not
sure, probably one of the C standards.) -D_FORTIFY_SOURCE=2 adds extra
checks above that. Some light testing hasn't shown any obvious problems.

Makefile.in

@@ -31,6 +31,11 @@ ZENMAPDIR = zenmap
 NDIFFDIR = ndiff
 PYTHON = @PYTHON@
 DEFS = @DEFS@ -DNMAP_NAME=\"$(NMAP_NAME)\" -DNMAP_URL=\"$(NMAP_URL)\" -DNMAP_PLATFORM=\"$(NMAP_PLATFORM)\" -DNMAPDATADIR=\"$(nmapdatadir)\"
+# With GCC, add extra security checks to source code.
+# 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
 # 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