linux: GlobalMenu: only if UBUNTU_MENUPROXY is set

When checking if we should react differently when GlobalMenu bar is
used, we should check if UBUNTU_MENUPROXY env var is (correctly) set
instead of checking if we're using Unity on a Unity session.

It's better to do that because we can use Compiz on a Unity session
without Unity. Or we can also use Unity in a different session.
Merge pull request #699 from matttbe/globalmenu

linux: GlobalMenu: only if UBUNTU_MENUPROXY is set