gh-110014: Fix _POSIX_THREADS and _POSIX_SEMAPHORES usage

[vstinner]

• pycore_pythread.h is now the central place to make sure that _POSIX_THREADS and _POSIX_SEMAPHORES macros are defined if available.
• Make sure that pycore_pythread.h is included when _POSIX_THREADS and _POSIX_SEMAPHORES macros are tested.
• PY_TIMEOUT_MAX is now defined as a constant, since its value depends on _POSIX_THREADS, instead of being defined as a macro.
• Document the change and give hints how to fix affected code.

• Issue: GCC warnings on pycore_semaphore.h and Python/parking_lot.c #110014

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📚 Documentation preview 📚: https://cpython-previews--110139.org.readthedocs.build/

[vstinner]

@encukou: "Oops", I have to add an exception to Tools/build/smelly.py to tolerate the new symbol PY_TIMEOUT_MAX. Before, Python only exported symbols starting with Py, _Py and __Py (macOS). Copy of my comment:

# "Legacy": some old symbols are prefixed by "PY_".EXCEPTIONS=frozenset({'PY_TIMEOUT_MAX', })

I would prefer to prefix all public symbols with Py and only have private symbols, but at least, now we have tooling to detect when new symbols with different prefix are added ;-)

[vstinner]

Oh. I don't know if it's related or not, but I had to write PR gh-110155 to add many <unistd.h>. WASM/WASI builds were badly broken :-(

[encukou]

@vstinner Could we instead exclude this undocumented constant from the limited API?

[vstinner]

I checked and apparently, it's safe to remove this constant, so I created PR gh-110217 for that.