macOS Support

[McCallisterRomer]

Just throwing this here so it exists somewhere

Proper support for mac can't happen until at least the next AppleClang release, maybe longer. The current version of AppleClang is 14, which roughly corresponds to clang 14. Adequate support for c++20 doesn't exist until clang 15, with 16 adding a few more things we're currently using. Ultimately, this only limits us from supporting AppleClang/Xcode - we are still free to add mac support with the caveat that a newer clang version must be installed.

I'm moving forward with clang 16 for now, as it will limit the total number of required changes. This will affect how we do CI builds as well, so I'm setting up my local environment to match GitHub runners (they have v15 now, but I'm just assuming we'll have 16 available soonish).

Get clang:
brew install llvm@16

Then provide compiler to configure env (of course, don't set generator to Xcode):
CXX=$(brew --prefix llvm@16)/bin/clang++