Add sdl as build platform but beware...

that I  could not test compile it since i dont know if its SDL1 or SDL2

Reasons are:
 * JGE.h uses SDLKey wich is SDL 1 (SDL_Keycode in SDL2)
 * GameOptions.cpp:1194 wont compile with SDLKey
     (because stringstream cant use >> on SDLKey but on SDL_Keycode)
 * changing the typdef in JGE.h and using SDL2 works fine for JGE
 * but SDLmain.cpp contains features from SDL2 (SDL_Window)
   and SDL1 (SDL_GL_SwapBuffers)