maint: make it easier to copy FORTIFY_SOURCE snippet

maint: make it easier to copy FORTIFY_SOURCE snippet

While libvirt intentionally avoids -Wundef (after all, C99
guarantees sane semantics of treating undefined macros as 0),
the glibc insanity of #warning on _FORTIFY_SOURCE coupled with
what some people feel is the black magic of autoconf means
that other projects are likely to copy our snippet verbatim.
We can be nicer to other projects by making it easier to
integrate into projects that use -Wundef.

Suggested by Christophe Fergeau.

* m4/virt-compile-warnings.m4 (LIBVIRT_COMPILE_WARNINGS): Be nice
to other projects using -Wundef.