That's a bug i've been telling the mac guy a few times but it never got fixed (i should have filed a bug report, i know!!)
Possibilities:
1) If you plan on installing GTK someday, do it now
2) Build from source (not macports) and edit the configure file in order to remove the bit that checks for GTK
3) File that bug report i was always too lazy to: "Mac version of C::B checks for GTK during configure while it doesn't use it at all"
4) Use a binary download