Support cross-compiling pets_lib for macOS

[Lamby777]

Tried this earlier and it was complaining about cc stuff and Xcode crap.

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Then I found this based individual's article.

Completely forgot Zig could do this sort of stuff. Gonna try messing around with it later, and hopefully I can get it to work on exporting a macOS binary. Won't be able to test it, though, unless I make a VM ( pls fuck off, Apple lawyers. I know you're reading this :3 ) or give it to someone else to try.

Not really a huge priority anyway. If they're making it this hard to ship basic stuff for their platform, I might as well not bother trying to support it. Still would be a plus if I could, though.