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There is the Julia package https://github.com/JuliaPackaging/BinaryBuilder.jl which creates an environment that fakes being another, but with the correct compilers and SDKs . It's used to build all the binary dependencies

For compiling, I'm pretty happy with https://github.com/tpoechtrager/osxcross.

For ardour we use QEMU VMs, running GUI-free for non-interactive builds, and with a GUI for ... interaction. Easy to create, and as long as you have lots of memory and cores, nice and responsive/fast. Set a startup item in the non-interactive VMs so that a minute or two after they come up, they start the build. In our case, they upload to ardour.org when complete. The interactive ones are just .. well, they're just macs running in their own window, do what you want with them.