Support for building with Emscripten

[parkertomatoes]

I updated this to run in the browser using Emscripten. It runs well on Chrome and Firefox for me

It still builds under MacOS X using the Linux Makefile (make -f Makefile.linux), but now it also builds with Emscripten (emmake make -f Makefile.linux).

Since the code uses blocking calls to SDL_Delay I built with Asyncify, and only a few minor changes were required.

• Calls to SDL_Delay were changed to emscripten_sleep to avoid blocking the main thread.
• Calls to exit were changed to emscripten_force_exit to avoid exceptions being thrown after exiting
• A hint function is called to keep Emscripten from resizing the canvas element.

All emscripten-specific changes, including in the Makefile, are made with a preprocessor switch (#ifdef __EMSCRIPTEN__), so the old functionality isn't affected.

UPDATE: Removed main loop refactor after realizing it wasn't necessary when using Asyncify

[xesf]

Merged your changes into my fork and made a release version of it: https://github.com/xesf/jc_reborn

[xesf]

This PR has been applied in these changes: #17