gh-129666: Add C11/C++11 to docs and -pedantic-errors to GCC/clang test_c[pp]ext tests

[encukou]

Ask GCC/clang to more strictly validate compliance with the standards.
Add opt-outs where currently necessary.

Add a C/C++ version note to docs.

This applies #130686 again, and then does a different dance for C++03. There, I couldn't find a way to keep GCC pedantic on the full API. For the limited API, we only need to remove one comma.

I'll run the buildbots this time.

• Issue: Document that public headers target C11 and C++11 #129666

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📚 Documentation preview 📚: https://cpython-previews--130692.org.readthedocs.build/

[chris-eibl]

clang (in contrast to gcc) suppresses both warnings

• comma at end of enumerator list
• ISO C++ 1998 does not support 'long long'

when -Wno-c++11-extensions is given. See https://godbolt.org/z/E1o1EfKWT.

If you can/want to differ between gcc and clang, you could run the non-limited build for clang, too.

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @encukou for commit 6c714df 🤖

Results will be shown at:

https://buildbot.python.org/all/#/grid?branch=refs%2Fpull%2F130692%2Fmerge

If you want to schedule another build, you need to add the 🔨 test-with-buildbots label again.

[encukou]

The buildbot failures are pre-existing.

[bedevere-bot]

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot x86 Debian Non-Debug with X 3.x (no tier) has failed when building commit d91cc9d.

What do you need to do:

1. Don't panic.
2. Check the buildbot page in the devguide if you don't know what the buildbots are or how they work.
3. Go to the page of the buildbot that failed (https://buildbot.python.org/#/builders/1245/builds/4490) and take a look at the build logs.
4. Check if the failure is related to this commit (d91cc9d) or if it is a false positive.
5. If the failure is related to this commit, please, reflect that on the issue and make a new Pull Request with a fix.

You can take a look at the buildbot page here:

https://buildbot.python.org/#/builders/1245/builds/4490

Failed tests:

• test.test_concurrent_futures.test_process_pool

Failed subtests:

• test_force_shutdown_workers - test.test_concurrent_futures.test_process_pool.ProcessPoolForkserverProcessPoolExecutorTest.test_force_shutdown_workers
• test_force_shutdown_workers - test.test_concurrent_futures.test_process_pool.ProcessPoolForkProcessPoolExecutorTest.test_force_shutdown_workers

Summary of the results of the build (if available):

==

Click to see traceback logs

Traceback (most recent call last):
  File "/buildbot/buildarea/3.x.ware-debian-x86.nondebug/build/Lib/test/test_importlib/metadata/fixtures.py", line 391, in _inner
    func(self, **args)
    ~~~~^^^^^^^^^^^^^^
  File "/buildbot/buildarea/3.x.ware-debian-x86.nondebug/build/Lib/test/test_concurrent_futures/test_process_pool.py", line 279, in test_force_shutdown_workers
    self.assertEqual(worker_process.exitcode, -signal.SIGTERM)
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: 255 != -15


Traceback (most recent call last):
  File "/buildbot/buildarea/3.x.ware-debian-x86.nondebug/build/Lib/test/test_importlib/metadata/fixtures.py", line 391, in _inner
    func(self, **args)
    ~~~~^^^^^^^^^^^^^^
  File "/buildbot/buildarea/3.x.ware-debian-x86.nondebug/build/Lib/test/test_concurrent_futures/test_process_pool.py", line 281, in test_force_shutdown_workers
    self.assertEqual(worker_process.exitcode, -signal.SIGKILL)
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: 255 != -9