Check that 'dl' function is available and usable before trying to call it with error...

Check that 'dl' function is available and usable before trying to call it with error suppression; if it's disabled or unavailable we end up with mysterious failures during installation or loading of libraries.

Fixed for StatusNet installer as well as some external libraries that should be fixed upstream if they haven't already been:
* PEAR
* Auth/OpenID
* Auth/Yadis