build: Add glib_debug option

To allow disabling debug infrastructure in builds with debug symbols.

docs/macros.txt

@@ -5,6 +5,10 @@ GLib's configure options and corresponding macros
 	none
 --buildtype={debug,debugoptimized}	[debugoptimized is the default]
 	-DG_ENABLE_DEBUG -g
+-Dglib_debug=disabled
+	Omits G_ENABLE_DEBUG when implied by --buildtype/-Ddebug
+-Dglib_debug=enabled
+	Defines G_ENABLE_DEBUG regardless of --buildtype/-Ddebug
 -Dglib_asserts=false
 	-DG_DISABLE_ASSERT
 -Dglib_checks=false