Ensure consistent enablement of gcc 'diagnostic' pragma

The virt-compile-warnings.m4 file would do an explicit
check for whether the compile could use the 'diagnostic'
pragma push/pop feature. The src/internal.h file would
then only enable it for GCC >= 4.6

This breaks with clang which supports the pragma but
does not claim GCC 4.6 compat. Export a variable from
the m4 check to the header file so they are consistent.
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>

m4/virt-compile-warnings.m4

@@ -94,6 +94,9 @@ AC_DEFUN([LIBVIRT_COMPILE_WARNINGS],[
       dontwarn="$dontwarn -Wmissing-prototypes"
       dontwarn="$dontwarn -Wmissing-declarations"
       dontwarn="$dontwarn -Wcast-align"
+    else
+      AC_DEFINE_UNQUOTED([WORKING_PRAGMA_PUSH], 1,
+       [Define to 1 if gcc supports pragma push/pop])
     fi
 
     dnl Check whether strchr(s, char variable) causes a bogus compile

src/internal.h

@@ -215,7 +215,7 @@
 # endif				/* __GNUC__ */
 
 
-# if __GNUC_PREREQ (4, 6)
+# if WORKING_PRAGMA_PUSH
 #  define VIR_WARNINGS_NO_CAST_ALIGN \
     _Pragma ("GCC diagnostic push") \
     _Pragma ("GCC diagnostic ignored \"-Wcast-align\"")