@deadpin, your suggestion to update XCode was the opposite of what I asked. It’s not necessarily a bag thing but it looked awfully like the behaviour of large industries in our society: if a consumer asks for a modification because of a problem, the producer offloads the responsibility onto the consumer and ignores the request. Example: all the big brands out there who produce hundreds of millions of plastic bottles and packaging, saying that since people buy them, they’re responsible for it. With respect to software development, engineers (I’m one of them) have a responsibility in making their software available to common hardware, because it’s not a sustainable approach to make people buy new hardware for every supposedly cutting edge feature.
And you might wonder why the ramble? Because I’ve got a macbook pro mid-2012 and I cannot update to any new MacOS as Catalina dropped support for it. So I can only install XCode 11.3.1. And if later another similar issue happens, this would mean “buy a new computer”. And if a developer can, with little efforts make that computer live longer for my use, I’ll push for it. Of course, I don’t like Apple’s way of doing business so I’ve got a second-hand workstation with Ubuntu and Windows as alternative for the day when I won’t be able to compile Blender anymore. 
Anyway @deadpin , I know you probably only wanted to help and you actually did so thank you for that. I hope you will understand why I mentioned this. And it’s clearly more Apple the culprit here when it comes to make things obsolete sooner than they should (but we all know they will do nothing to change that).
Meanwhile, I updated XCode to 11.3.1 and compilation works but linking still fails as I reported in a previous post and as @georgevdd said. There is no solution yet except downloading and using (at your own risk) the precompiled python 3.9 lib provided by @ankitm here: Login
Btw, if you update or downgrade XCode, there is a systematic import error with al.h for OpenAL at compilation time. This is solved by removing build_darwin/CMakeCache.txt before make.
Thanks @jacqueslucke for looking at this. If your target is MacOS 10.13, such users won’t be able to update to XCode 11 to solve the issue, so I guess it’s good that you found an alternative for std::variant and std::any.
I tested with both XCode 10.03 and 11.3.1, compilation succeeds, except linking as mentioned above.