gh-141563: Fix test_cppext on macOS

[vstinner]

Don't test internal header files including mimalloc on macOS since mimalloc emits compiler warnings:

In file included from extension.cpp:21:
In file included from Include/internal/pycore_backoff.h:15:
In file included from Include/internal/pycore_interp_structs.h:15:
In file included from Include/internal/pycore_tstate.h:14:
In file included from Include/internal/pycore_mimalloc.h:43:
Include/internal/mimalloc/mimalloc.h:464:85: error: defaulted
function definitions are a C++11 extension
[-Werror,-Wc++11-extensions]
mi_stl_allocator() mi_attr_noexcept = default;
^
Include/internal/mimalloc/mimalloc.h:465:85: error: defaulted
function definitions are a C++11 extension
[-Werror,-Wc++11-extensions]
mi_stl_allocator(const mi_stl_allocator&) mi_attr_noexcept = default;

Log also CXX and CXXFLAGS env vars in test_cppext. Log also CPPFLAGS in test_cext.

• Issue: The datetime C API is not thread-safe #141563

[vstinner]

!buildbot x86-64 MacOS Intel NoGIL

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @vstinner for commit ac9f7b0 🤖

Results will be shown at:

https://buildbot.python.org/all/#/grid?branch=refs%2Fpull%2F144685%2Fmerge

The command will test the builders whose names match following regular expression: x86-64 MacOS Intel NoGIL

The builders matched are:

• x86-64 MacOS Intel NoGIL PR

[vstinner]

"buildbot/x86-64 MacOS Intel NoGIL PR" passed successfully.