Bump to Love 11.4

[alexjgriffith]

1. Applied changes made to Love 11.3 for emscripten compatibility to version 11.4.
2. Tested the compat and release targets.
3. Added a small enviro for debugging the build (uses the appropriate headers for the release target)
4. Generalized build_lovejs.sh. You now pass in your paths to the megasource and emsdk
5. Updated the readme to clarify that emsdk V. 2.0.0 works when compiling (likely need to update game.js to allow for compatibility with the latest version of emscripten)

Closes #53 and #56

[Davidobot]

Looks good! Thanks for doing this!

[Davidobot]

@alexjgriffith could you please push the appropriate changes to https://github.com/Davidobot/love/tree/emscripten ? (and https://github.com/Davidobot/megasource/tree/emscripten if necessary)

I want to take a crack at implementing #2 (comment)

[alexjgriffith]

I've created pull requests in your emscripten branch for both Love and the Megasource. I'd recommend creating separate branches for each version of Love that we directly support (So far 11.3 and 11.4)