define ('MESSAGE_WINDOW', true); is defined in message/lib.php which is not correct because it is defined in bunch of other files which causes:
Notice: Constant MESSAGE_WINDOW already defined in /server/workspace/moodle20/message/lib.php on line 30
In any case using define for prevention of message popups does not seem like a good idea. This looks like a typical properly of $PAGE, it should not be very difficult to add support for it there. Or tehre might be some other better solution.