Do not require a config.php for xcache-admin

[pierres]

Some variables like $enable_eval are accessed even though they are only defined in config.php.example.

Adding a isset() check before accessing such variables would make the config file optional and wont rise any PHP warnings.

This would make it a little easier to directly use the xcache admin distributed with the package.

[moo]

fixed in [902]. config is now changed