The AmigaOS code has been removed from SDL, since it had been broken for a
 long time and had a few bits of fairly invasive code #ifdef'd into the
 SDL core.

However, there is an OS4 version of SDL here:
   http://www.rcdrummond.net/amiga/index.html

And a MorphOS version here:
   http://www.lehtoranta.net/powersdl/

--ryan.