MythMusic: show a popup if Rip CD is selected but support isn't compiled in

There's at least one distro shipping MythMusic without libcdio support
compiled in which means CD ripping isn't available. This just shows a
message telling the user why they can't rip a CD.

Should really look at hidding the Rip CD menu item if support for it isn't
available but that's something we can't do at the moment.