On Windows a successful `sched_yield()` detection requires mingw-w64
built with POSIX threads (not Win32 threads) and GCC (not llvm/clang).
(linking to `winpthread` via custom options may also work.)
In CMake builds, it was pre-cached as unavailable before this patch.
When detected (via autotools), it got only used for Windows XP or older
targets combined with a non-GCC, non-clang compiler that doesn't support
`__builtin_ia32_pause()`, or with the Intel C compiler. According to
`lib/easy_lock.h`.
mingw-w64 only supports GCC and clang, leaving a very narrow chance when
`shed_yield()` gets called on Windows. Even then, `sched_yield()` is
implemented in `winpthread` as `Sleep(0)`, which may or not be a useful.
It's also trivial to implement locally if it is, and such rare build
combination is also deemed useful.
Thus, this patch marks `sched_yields()` permanently unavailable on the
Windows platform also with autotools, and instead of pre-caching, skip
this feature check with CMake.
This syncs `HAVE_SCHED_YIELDS` between builds methods on Windows.
Follow-up to 9b517c8b69#11973
Follow-up to 23af112f55#8680Closes#16037
Replace `strtok_r()` detection with detection mingw-w64 version.
The use this version to pre-fill `HAVE_STRTOK_R`.
This gives use mingw-w64 version information for free.
Closes#16022
`stdatomic.h` and `_Atomic` were first available in gcc 4.9.0 and
llvm/clang 3.6. Set detection values accordingly and save these two
detections on configure runs.
Closes#16036
- cmake: add auto-detection. Sync this with autotools.
- enable for MS-DOS and AmigaOS builds.
(auto-detection doesn't work for cross-builds.)
- tidy up detection snippet.
- fix comment.
Closes#15868
