Check dpkg-vendor to set the PHP_BUILD_PROVIDER value

Support the PHP_BUILD_PROVIDER flag in Ubuntu as well, checking dpkg-vendor and setting it accordingly.

This is the same diff applied to the Ubuntu package. While we could have hardcoded it to Ubuntu there and call it a day, checking it here reduces the delta, which we would appreciate (:

This check is being done in a similar way in other packages, such as dovecot, mysql-defaults, autofs and others.

Please let me know if this is acceptable. Also, as this is my first code contribution, please let me know if opening MRs as this one is fine, or if I should open a bug, send a patch, or anything you would prefer.

Thanks!