I personally sniffed around this SDL problem, and maybe I know what the problem is.
For SDL 1.2, it still depends on NSQuickDrawVIEW (AppKit/NSQuickDrawView.h), which is now deprecated, replaced by Quartz and removed from 10.6, thus is unable to compile.
Also, now-removed AUNTComponent (AudioUnit/AUNTComponent.h) for older AudioUnit specifications is required for SDL 1.2.
For SDL_mixer, it depends on QuickTimeMusic (QuickTime/QuickTimeMusic.h), which is not supported for 64-bit development (still available for 32-bit development), thus is unable to compile.
Workaround is to use MacOSX10.5.sdk or earlier (-isysroot <sdk> to gcc), and don't compile these for 64-bit support (gcc-4.0, not gcc-4.2).
I'm looking forward to see SDL 1.3, but I know some functionalities are not fully working (e.g. alpha blending).