The first thing you have to do when porting to a new platform, is look at include/SDL_platform.h and create an entry there for your operating system. The standard format is "PLATFORM", where PLATFORM is the name of the OS. Ideally SDL_platform.h will be able to auto-detect the system it's building on based on C preprocessor symbols.
There are two basic ways of building SDL at the moment:
If you have a GNUish system, then you might try this. Edit configure.ac, take a look at the large section labelled:
"Set up the configuration based on the host platform!"
Add a section for your platform, and then re-run autogen.sh and build!
If you're using an IDE or other non-configure build system, you'll probably want to create a custom SDL_config.h for your platform. Edit SDL_config.h, add a section for your platform, and create a custom SDLconfig{platform}.h, based on SDL_config_minimal.h and SDL_config.h.in
Add the top level include directory to the header search path, and then add the following sources to the project:
src/*.c
src/atomic/*.c
src/audio/*.c
src/cpuinfo/*.c
src/events/*.c
src/file/*.c
src/haptic/*.c
src/joystick/*.c
src/power/*.c
src/render/*.c
src/render/software/*.c
src/stdlib/*.c
src/thread/*.c
src/timer/*.c
src/video/*.c
src/audio/disk/*.c
src/audio/dummy/*.c
src/filesystem/dummy/*.c
src/video/dummy/*.c
src/haptic/dummy/*.c
src/joystick/dummy/*.c
src/main/dummy/*.c
src/thread/generic/*.c
src/timer/dummy/*.c
src/loadso/dummy/*.c
Once you have a working library without any drivers, you can go back to each of the major subsystems and start implementing drivers for your platform.
If you have any questions, don't hesitate to ask on the SDL mailing list:
http://www.libsdl.org/mailing-list.php
Enjoy!
Sam Lantinga (slouken@libsdl.org)