build: ask for -std=gnu99 explicitly

We previously got -std=gnu99 secretly enabled as a side-effect
of requesting the 'stdarg' gnulib module. We rely on some
extensions from c99/gnu99 and while RHEL-7 supports this, it
still defaults to gnu89.  RHEL-7 also supports some newer
standards but declares them experimental/incomplete, so sticking
with gnu99 is best bet for now & matches historical usage.
Reviewed-by: NFabiano Fidêncio <fidencio@redhat.com>
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>

m4/virt-compile-warnings.m4

@@ -189,6 +189,11 @@ AC_DEFUN([LIBVIRT_COMPILE_WARNINGS],[
       wantwarn="$wantwarn -Werror"
     fi
 
+    # Request the gnu99 standard which is the best choice with
+    # gcc 4.8.0. Not a warning flag, but the probing mechanism
+    # is convenient
+    wantwarn="-std=gnu99"
+
     # Check for $CC support of each warning
     for w in $wantwarn; do
       gl_WARN_ADD([$w])