Impossible, legally, since you aren't licensed to install OS X headers and libraries even for cross-compilation on other OSes. You could generate object code (if you can work around not having OS includes, or you don't need them) but final link would need to be done on a Mac to comply with Apple licensing. (Well, since Apple uses dynamic linking exclusively, in theory someone could clean-room enough of a fake e.g. libSystem.dylib to enable cross-link without actually having any of its object code. Lot of work for dubious amount of gain, though, and the edge cases would be horrendous.)
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