Alter the way we handle compression within mantis:

This only applies zlib is enabled

if zlib.output_compression is not enabled in php.ini we use ob_gzhandler, else we use output_compression

Note: we behave differently if php is >=5.2.10 as we can use zlib.output_compression with ini_set: Note php manual:
"Note: You cannot use both ob_gzhandler() and zlib.output_compression. Also note that using zlib.output_compression is preferred over ob_gzhandler(). "

COMPRESSION_DISABLED can be defined as a constant to disable compression on a specific page if required, before including core.php.