Add support for Emscripten

This patch series adds support for Emscripten as a target for Regal.  It successfully builds and runs as a static library that can then be linked in to GL-using projects.  (There's no real "shared library" in the emscripten world, so static builds are all that's necessary.)

Many of the same limitations of nacl apply to emscripten/webgl, so the code takes similar paths.  This doesn't add any functionality (with the exception of the GL_FOG* getters that I needed for a specific project I was using to test).

For desktop, I tested linux-64 builds to make sure they didn't regress, but wasn't able to test many others.  Most of the code is behind ifdefs though, so I suspect that there should be no issues.  The toplevel Makefile changes could be cleaner; introducing a variable that means nacl-or-emscripten would help improve things.