Linux port using SDL

This port is based on SDL, SDL_mixer, OpenAL and libpng.

It compiles on Debian Squeeze with the following versions:

* gcc - 4.3.5
* SDL - 1.2.14
* SDL_mixer - 1.2.8
* OpenAL - 1.12
* libpng - 1.5.8

I borrowed the PNG loading code as well as libpng from the OS X implementation.