...I had to add
#include <SDL_thread.h>
Thanks, I added this to types.h. I guess that the SDL on my machine includes SDL_thread.h when I include SDL.h, and maybe either the 32 bit version doesn't or it's a difference in the SDL version. I'm using amd64 Gentoo and I have media-libs/libsdl-1.2.13 installed. Either way, there shouldn't be any harm in attempting to include it twice.
Also, this answers my question from the other thread, you are running 32 bit linux.
I have another machine here I can install that on, maybe I'll do that this next weekend.