Meson: Add glib_checks and glib_asserts options

In cases where performance are critical it can be useful to disable
checks and asserts. GStreamer has those options too, using the same name
and setting them yielding means we can set those options on the main
project (e.g. gst-build) and glib will inherit the same value when built
as subproject.

meson_options.txt

@@ -94,3 +94,15 @@ option('oss_fuzz',
        type : 'feature',
        value : 'disabled',
        description : 'Indicate oss-fuzz build environment')
+
+option('glib_assert',
+       type : 'boolean',
+       value : true,
+       yield : true,
+       description : 'Enable GLib assertion (see docs/macros.txt)')
+
+option('glib_checks',
+       type : 'boolean',
+       value : true,
+       yield : true,
+       description : 'Enable GLib checks such as API guards (see docs/macros.txt)')