meson: Default to gnu99

We don't really require all of C99, but enough that it's not worth
bothering with the distinction, especially if your toolchain is new
enough that meson is a thing for you. We could do strict C99 if we
really insisted on spelling it __typeof__, but who wants that? Nobody,
that's who.

Signed-off-by: Adam Jackson <ajax@redhat.com>
Reviewed-by: Eric Anholt <eric@anholt.net>

meson.build

@@ -1,5 +1,8 @@
 project('xserver', 'c',
-        default_options: ['buildtype=debugoptimized'],
+        default_options: [
+            'buildtype=debugoptimized',
+            'c_std=gnu99',
+        ],
         version: '1.19.99.1',
         meson_version: '>= 0.40.0',
 )