I have never cross-compiled from Windows/Linux => OSX but given my experience going the other way, I will recommend first finding an existing C/C++ compiler setup that does that for you before trying within Nim.
[https://github.com/tpoechtrager/osxcross](https://github.com/tpoechtrager/osxcross) is the only real link I have found and it is only for Linux => OSX. Source: [https://stackoverflow.com/questions/693952/how-to-compile-for-os-x-in-linux-or-windows/19891283#19891283](https://stackoverflow.com/questions/693952/how-to-compile-for-os-x-in-linux-or-windows/19891283#19891283)
