libretro: Add the standard Makefile

[RobLoach]

@nwhitehead I was able to make it build with libretro's standard Makefile. This provides all the configuration needed to build across the different platforms that libretro's buildbot has defined. Ran into a problem with the bundler though. I'm unsure we'd be able to run the Python/Node.js bundler on the buildbot.

Would it be okay if we commited the bundled files directly into the repository?

[nwhitehead]

@RobLoach Yeah, committing the bundled file is the right approach. It should be entirely platform independent.