@SpecLad  I have been wondering whether my approach to this was the best one for a little while now. I have just been looking into this and would appreciate your thoughts.
My proposal is to make WITH_GTK include gtk3 by default, and use WITH_GTK_2_X as a second option (always visible when WITH_GTK is visible) to select gtk2.x instead of gtk3. This is more consistent with the approach taken for gstreamer support and should therefore be less confusing. Does this sound like a reasonable approach?