Use XDG_CURRENT_DESKTOP to detect GNOME desktop like systems
Use XDG_CURRENT_DESKTOP to detect GNOME desktop like systems

Fixes: https://github.com/libproxy/libproxy/issues/191

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