Fix WITH_GNOME2 documentation
Fix WITH_GNOME2 documentation

The option is "WITH_GNOME2" not "WITH_GNOME", and it defaults to OFF.

libproxy/libproxy,libproxy/libproxy,libproxy/libproxy,libproxy/libproxy,libproxy/libproxy,libproxy/libproxy,libproxy/libproxy