It would be very useful for the build team if Julia itself could be cross-compiled. Among other things, this would enable building Julia under BinaryBuilder, a long-standing desire of myself and anyone else that has to maintain Julia-building infrastructure.

The fundamental limitation is, of course, Julia's bootstrap process. We can cross-compile the C parts just fine, but the bootstrap of Base and stdlib are not cross-compilable yet. On the other hand, as far as I can tell, bootstrap doesn't actually do that much introspection of the system it's running on, so it seems reasonable to me that we could pass some kind of -C x86_64-apple-darwin14 and generate a sys.ji/sys.o for that particular target.

We need to:

• Tell LLVM to codegen for the appropriate architecture/OS calling conventions, etc....
• Tell Julia/LLVM to output code in the appropriate format (e.g. .dylib/.dll/.so)
• Force the bootstrap process to build build_h.jl appropriately so that it thinks it is in fact on Windows, etc... (This might be already done through BB's mocking of uname, I haven't checked)