Inspired by #504.
Instead of requiring the user to do PLATFORM_ANDROID #ifdefery,
have the android_main entry point exported by raylib and call
the user-defined main. This way many games could (in theory)
run unmodified on Android and elsewhere.
This is untested!
They were disabled because they failed to build,
but this patch set fixes the build on Linux and macOS.
This doesn't apply to the AppVeyor build on Windows yet;
it currently fails at linking with OpenAL.
