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.

docs/macros.txt

@@ -5,9 +5,10 @@ GLib's configure options and corresponding macros
 	none
 --buildtype={debug,debugoptimized}	[debugoptimized is the default]
 	-DG_ENABLE_DEBUG -g
-
-Available to define yourself:
-	-DG_DISABLE_ASSERT -DG_DISABLE_CHECKS
+-Dglib_asserts=false
+	-DG_DISABLE_ASSERT
+-Dglib_checks=false
+	-DG_DISABLE_CHECKS
 
 Besides these, there are some local feature specific options, but my main
 focus here is to concentrate on macros that affect overall GLib behaviour