Clean up debug features

Introduce a GTK_DEBUG_CHECK() macro and use it to check for
GTK_DEBUG flags everywhere. Also guard all such places by

gtk/gtkbuilderparser.c

@@ -926,7 +926,7 @@ start_element (GMarkupParseContext  *context,
   ParserData *data = (ParserData*)user_data;
 
 #ifdef G_ENABLE_DEBUG
-  if (gtk_get_debug_flags () & GTK_DEBUG_BUILDER)
+  if (GTK_DEBUG_CHECK (BUILDER))
     {
       GString *tags = g_string_new ("");
       int i;