You can't have options depend on each other. Well, I mean you can, but then the dependent option won't be visible in the GUI until the user reruns the configuration step, and I think that's bad design. (You can see that the `WITH_GSTREAMER_1_X` option is unconstrained.) The rest of your plan I'm okay with, although I think if GTK+ 3.x is not found, it should try 2.x as a fallback, even if ``WITH_GTK_2_X` is off (again, GStreamer support does that).