For native files we can do more minimal fixes to avoid this large of a
hammer, but for third party files it may not be worth the effort to try
and patch them. NetBSD has the original _FORTIFY_SOURCE implementation
that ours is based on, for instance, but tests sourced from there can't
do an __ssp_real(foo) without being certain that foo actually has a
fortified definition.

This change does always define _FORTIFY_SOURCE as a result, so gate it
on CFLAGS not already containing _FORTIFY_SOURCE definitions.

PR: 294881