Makefile: use default $CC instead of only gcc

Using $CC, but only after unconditionally setting it to gcc, is kind of
silly.
Update README.md for emscripten
Remove OpenMP support

Emscripten doesn't support OpenMP.
Drop SDL_EventState() calls

These don't really do a whole lot, and cause Emscripten problems (SDL
1.3 vs 2.0), so drop 'em.
Use internal linkage: s/^inline/static inline/

Emscripten has problems with just "inline" functions, but seems to
accept "static inline".
Implement our own RNG: lame_rand()

Emscripten doesn't seem to like lrand48() very much, so we implement our
own random number generator. Note that we only use one bit from the RNG,
and we want it to be a good bit, so we shift the output of lame_rand()
before returning it.
Break out main loop to game_iter()

Quick 'n dirty: move important variables out to globals, and move inside
of while loop out to game_iter() function.
Add actual Emscripten support

Use #ifdef-s to separate Emscripten code from native.

Also lock/unlock surface before writing, since it doesn't work for me
otherwise.
Shrink from 1280x720 to 640x360 if emscripting

Javascript is still slower than native, especially without OpenMP
support.
Fix typos in README.md

s/Game_of_Life/Game_of_Ice/i