>> 1) __APPLE_USE_RFC_3542 should have been in socketmodule.c from the start, not sure why it was added in setup.py.

> FTR, it was added by me in bpo-35569, GH-19526. I moved the check to setup.py because Ned made me do it :P

Yeah, well, it seemed like a good thing at the time :)

> Something you don't mention is the logic dealing with SDK roots. I haven't checked yet if similar logic would be necessary in configure. With some luck it isn't, but that depends on what's supported by autoconf and the particular probes we want to use.

I haven't looked closely at it in this context but I believe most, if not all, of the SDK root shenanigans in setup.py shouldn't be necessary in autoconf tests where the Apple compiler driver handles the SDK details and the few other cases, like for when there was only a 32-bit version of Tk for macOS, are no longer relevant. In any case, that's something for us to follow up on once Christian's changes are merged and stabilized on other platforms.

> @Ned: Not being able to use system versions of libraries is a bit annoying, but that's something you already have to do due to openssl.  Maybe we should also try to clean up and refactor the build-installer.py script to do have a way to build/install those 3th-party dependencies.

Yes. I want to pull the third-party lib building out of build-installer.py into a simple separate build step, probably just using make, that can produce the minimum sets of third-party libs for use by both installer builds and by developers so that there is no dependency on other third-party distributors like Homebrew or MacPorts. With a little bit of care, it wouldn't have to be limited to just macOS builds.