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.