Discussion regarding creating an open macOS SDK package.

I think no one wants to tell people to use old macOS's or download stuff from people with hackery sounding usernames on github, at least I do not.

Now that https://github.com/conda-forge/staged-recipes/pull/9557 has happened, we should be able to allow end-users to use their system SDKs via `CONDA_BUILD_SYSROOT`, but that's not ideal for ABI reasons.

But I think we should provide our own SDKs. The official SDKs are an amalgamation of various mostly BSD licensed projects (mach-o kernel, dyld, a few more). I think we can now generate .tbd files from Apple's .dylibs and we can also 'whiteroom' that if people are concerned - you literally get one person to describe, via voice, the functions and their prototypes! - but personally I'm not really very concerned. So I think, given a simple bash script we could make something we can use and recommend to our users.

cc @conda-forge/core, @chrisburr (you're in core anyway I guess, but just in case), @katietz, @msarahan.