Address macOS compatibility warnings

[colinsullivan]

If you look at the logs of the macOS 14 arm with llvm 19, what does the warnings at the end of the build step mean? (Object files built for newer->14 than linked ->10.15)
Is it problematic?

hmm I'll check this out - usually this is related to versioning of macOS and their supported APIs - I would think for our purposes it does not matter but could lead to compatibility issues on old macOS systems

having a consistent CMAKE_OSX_DEPLOYMENT_TARGET for every target being built should resolve this setting. If you are supporting ARM, then you cannot go lower than macOS 11.0

I believe this warning could lead to crashes if you use new APIs on an older OS -- the macOS SDK will shim or replace OS calls that do not exist on older OS's when building, but does nothing if your API level and deployment target match.

How much this matters for the llvm codebase, I have no idea

Originally posted by @kevin-- in #82 (comment)