I just had a very similar problem with OpenSuSE 10.2 AMD64. Installed all of the SDL devel libraries, but still could not find SDL_image.h or SDL_mixer.h. These are apparently not included with the DVD for some reason. I was able to download the source libraries from
http://www.libsdl.org/projects/SDL_image/ and
http://www.libsdl.org/projects/SDL_mixer/. ./configure, make, and make install worked fine with these, but Sauerbratten still wouldn't find the header files.
So I added the base path for these (/usr/local/lib) into /etc/ld.so.conf as requested by these files when I installed them. Still no luck. I poked around and discovered that the header files are not in /usr/local/lib/ but rather in /usr/local/include/SDL/
So I cd'd into the Sauerbratten src directory and created symbolic links to the files:
ln -s /usr/local/include/SDL/SDL_image.h SDL_image.h
ln -s /usr/local/include/SDL/SDL_mixer.h SDL_mixer.h
make install
Then it worked like a champ, and unlike with most Linux build from source type installations, it only took a couple of hours trying to track down the little problems. I'm not sure about the header files above, though -- where should they be to work properly in future builds?