So darwin.mk from HEAD (and as far back as I can remember offhand) has a nice, convenient little section where it checks to see if /opt/local or /sw exist and, if so, add the first one to exist to CFLAGS/LDFLAGS. This caused some troubles for me back in the day because instead of having my extra libs/headers in either of those locations, I had them in /usr/local (shocking, I know).
Once I figured out what was going on it was a quick fix. Over time, though, UFO:AI has picked up a few more dependencies, some of which have installed themselves in other places—the most recent being, in fact, in /opt/local.
Is there in fact a realistic fear of people getting stale libraries/headers from the wrong source if, instead of selecting precisely one source for headers/libs, darwin.mk instead tested for existence of common places to stick such things and then simply added each one it found (plus /usr/local and /Library/Frameworks) to CFLAGS/LDFLAGS appropriately? It would require a few more tests to tease out exactly where SDL_* is living, but not much more, and it seems like it would dramatically reduce the likelihood that someone will need to edit darwin.mk (or, I guess, probably pass the right options to configure) to have a successful build.
~J
