Adds build support for emscripten (js output)
Forces glew to not compile through emscripten and fudges SOIL so that it
does.

I might need to look to SDL_image instead of SOIL or figure out how to
properly port SOIL.
Moves all sdl/gl includes to backend/sdl.h for emscripten.
Compiles under emscripten to produce html/js.
Ignores various build relics.