Older gcc warns (on every file!) that -Wabi and -Wdeprecated only
make sense on C++ projects.  Newer gcc accepts these warnings for
C, but it is not clear that they can do anything useful, so it
is easier to just drop the warnings altogether.

* m4/virt-compile-warnings.m4 (LIBVIRT_COMPILE_WARNINGS): Silence
-Wabi and -Wdeprecated on older gcc.
Reported by Peter Krempa.