build: reduce warnings from older gcc

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.

m4/virt-compile-warnings.m4

@@ -22,6 +22,8 @@ AC_DEFUN([LIBVIRT_COMPILE_WARNINGS],[
 
         # Don't care about C++ compiler compat
         dontwarn="$dontwarn -Wc++-compat"
+        dontwarn="$dontwarn -Wabi"
+        dontwarn="$dontwarn -Wdeprecated"
         # Don't care about ancient C standard compat
         dontwarn="$dontwarn -Wtraditional"
         # Don't care about ancient C standard compat