build: Always pass --quiet to g-ir-scanner

This makes the build less verbose, as all .gir generation except for
clutters didn't pass --quiet to g-ir-scanner, making it output long
linking commands. Do this by adding a common introspection_args
variable.

While at it, put -U_GNU_SOURCE in there too, as it was always passed
everywhere as without it the scanner would log warnings.

src/meson.build

@@ -820,9 +820,7 @@ if have_introspection
       mutter_deps,
       libmutter_dep,
     ],
-    extra_args: mutter_c_args + [
-      '-U_GNU_SOURCE',
-    ],
+    extra_args: mutter_c_args + introspection_args,
     install_dir_gir: pkglibdir,
     install_dir_typelib: pkglibdir,
     install: true