SPSCQueue: hack around libc++ bug on macOS

[Be-ing]

No description provided.

[Holzhaus]

Could you post the fix upstream, so that we can just update our library copy?

[Be-ing]

I don't know if that would be appropriate. The bug was in LLVM's libc++ and has already been fixed. SPSCQueue is doing it right according to the C++ standard.

[uklotzde]

This custom workaroundhack is not suitable for an upstream fix. Are we able to restrict it to selected LLVM/Clang versions?

[uklotzde]

Still unimplemented, so no need for a more specific check:
https://libcxx.llvm.org/docs/FeatureTestMacroTable.html

[Be-ing]

There are macros for the clang version number, but I do not know how to get the libc++ version. I am unclear if each version of clang must be used with a certain libc++ version or if they are independent.

[Be-ing]

CI passed. Merge?

[uklotzde]

Hopefully we don't forget to re-add this patch when updating from upstream. LGTM

[Be-ing]

Hopefully we don't forget to re-add this patch when updating from upstream.

CI will catch this quickly if we do forget it.