gettext: Guard against compiler not supporting __builtin_stpncpy

From https://lists.macosforge.org/pipermail/macports-dev/2011-July/015263.html:

5) Building with a compiler that doesn't support newer __builtins

If your port uses MacPorts compilers rather than the default compiler,
you may run into trouble with string functions.  You'll see errors at
link time about undefined __builtin_* functions.  If this happens, you
may want to compile with -D_FORTIFY_SOURCE=0 to tell the headers to
use unfortified versions which do not use compiler builtins.

pkgs/development/libraries/gettext/default.nix

@@ -19,6 +19,9 @@ stdenv.mkDerivation (rec {
             "--config-cache"
           ]);
 
+  makeFlags = stdenv.lib.optionalString stdenv.isDarwin
+    "CFLAGS=-D_FORTIFY_SOURCE=0";
+
   # On cross building, gettext supposes that the wchar.h from libc
   # does not fulfill gettext needs, so it tries to work with its
   # own wchar.h file, which does not cope well with the system's