Let me clarify: Unity gives you high-level APIs for things like graphics, audio, input, networking, that work across all supported platforms automatically. There may be some platform-specific quirks, but roughly speaking, you can just click "build for X" and get a working X build. There's a huge gap between compilation and that.
I don’t know how Jai will do this. But my gut feeling says that the meta programming capabilities will could help doing this sort of thing. My understanding is that it enables one to say „build my program in this particular way“ expressed within Jai itself.
I honestly think this is outside the scope of a language; it's really a library/framework/toolset thing. I'm sure it will be possible to do this with Jai (maybe it already is), but you'd still have to do it with Jai as opposed to Jai doing it.
