gh-116869: Add test_cext test: build a C extension

[vstinner]

• Issue: Python 3.12.2 headers contain non-C90-compliant code #116869

[vstinner]

Once my fix PR gh-116950 will be merged, I will write a second change on top of this issue to run tests with -Werror=declaration-after-statement to avoid regressions.

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @vstinner for commit d024b10 🤖

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

[vstinner]

Oh, the test catched a warning when using -std=c99:

      In file included from extension.c:7:
      In file included from /buildbot/buildarea/pull_request.ware-freebsd/build/Include/Python.h:122:
      /buildbot/buildarea/pull_request.ware-freebsd/build/Include/cpython/optimizer.h:77:3: error: redefinition of typedef '_PyOptimizerObject' is a C11 feature [-Werror,-Wtypedef-redefinition]
      } _PyOptimizerObject;
        ^
      /buildbot/buildarea/pull_request.ware-freebsd/build/Include/cpython/optimizer.h:60:35: note: previous definition is here
      typedef struct _PyOptimizerObject _PyOptimizerObject;
                                        ^
      1 error generated.

I wrote PR #116963 to fix it.

[vstinner]

PR rebased on top of PR #116963 merged.

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @vstinner for commit 8a5b224 🤖

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

[colesbury]

It should work now that #116878 landed!

(The comment was a bit inaccurate: it turns out the issue was in wheel, not pip)

[vstinner]

That's great! I will write a follow-up to patch also test_cppext

[vstinner]

PR #116973

[vstinner]

Merged. Let's start with something simple.