meson: Remove -std=c99

This flag disable some compiler feature which is used by some system header
potentially introducing some limitations.
Autotools won't add any flag to limit compiler features to C99, instead it
currently only add flags to support C99 when needed.
For instance some Posix limitations changes (like _POSIX_OPEN_MAX).
As compiler feature for instance _Static_assert is not used.

Signed-off-by: Frediano Ziglio <fziglio@redhat.com>
Acked-by: Christophe Fergeau <cfergeau@redhat.com>

meson.build

@@ -10,8 +10,7 @@ if not meson.is_subproject()
 endif
 
 # some global vars
-spice_common_global_cflags = ['-std=c99',
-                              '-DHAVE_CONFIG_H',
+spice_common_global_cflags = ['-DHAVE_CONFIG_H',
                               '-DG_LOG_DOMAIN="Spice"',
                               '-Wall',
                               '-Wextra',