Like commit 91f6460aff which added support
for Windows, this commit does the same for our final major OS.
The Darwin kernel exposes a set of __ulock_{wait,wait2,wake} APIs [1],
but these APIs are marked as private, so we cannot rely on them being
stable, nor we can use these APIs in builds of Qt intended for
the Apple App Store. By wholesale disabling the use of the APIs
in App Store compliant builds, and runtime checking availability
of the APIs when we do build them in, we should be safe, unless
the semantics of the APIs change in ways we haven't accounted for,
but that's a risk we're willing to take.
Note that libc++ uses these private APIs to implement P1135R5
(the C++20 synchronization library) [2], but that use is under
a "special permission" from the Darwin team, and meant only for
use in the Apple vendored version of libc++ shipped with their
operating systems, where they control both the kernel and the
standard library.
[1] https://github.com/apple-oss-distributions/xnu/blob/xnu-8792.81.2/bsd/sys/ulock.h
[2] https://reviews.llvm.org/D68480.
Change-Id: Ib709fc1585f647a98d54fffd16663b4965458404
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
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