fixed bug in fetch.php on windows systems

fetch.php used is_executable() to check for the ImageMagick convert
tool. The function is not available prior PHP 5 for Windows systems.
fetch now only checks if the config variable is set.

darcs-hash:20050922165540-7ad00-f787b5be5c4e9ce4eb31399a1f3ce8dcd1654c60.gz