For GTK2, I have restored the default behavior of installing for X11.
At some point it would be nice to be able to provide a nice transition from using X11 as the default for GTK to using Aqua as the GTK default, but we are not at that point yet (I thought we were closer than I now think we are to that point).
There are three possible installation states for GTK2:
gtk2+quartz with X11 on the Mac
gtk2+quartz without X11 on the Mac
State 1 is the state that almost all upstream developers expect, State 3 works if no X11 APIs is used in upstream development other than gtk+, and State 2 just simply breakslots of stuff horribly. Unfortunately, since OS X
10.5 ships with X11 installed by default, State 3 is not easily reached on 10.5, leaving State 1 as the only reliably working state.
Hence the decision to make that the default state.