Merge with pypa/distutils@22b9bcf

[jaraco]

• Suppress warnings in deprecated msvc compilers
• Fix, again, finding headers during cross compiling
• Rename _mangle_base to _make_relative and add documentation about its purpose. Ref New test_set_include_dirs leaves files in working directory distutils#169.
• In _make_relative, remove deprecation warning. Ref New test_set_include_dirs leaves files in working directory distutils#169.
• Extract method for _make_out_path.
• In _msvccompiler, only override _make_out_path
• In _msvccompiler, re-use _make_relative.
• Extract property for mapping src extensions to out extensions.
• Extract property for out_extensions in _msvccompiler
• Deprecate bcppcompiler. Support for Borland C++ compiler was dropped for compiling Python in Drop support of Borland C compiler python/cpython#66782 (2014) and the borlandc.org web site no longer references a compiler. Best I can tell, this compiler hasn't been released for 22 years. Surely no one is using it.
• Remove _msvccompiler._make_out_path.
• In cygwincompiler, re-use object_filenames from ccompiler.
• Add unit tests capturing the expectation

Summary of changes

Closes

Pull Request Checklist

• Changes have tests
• News fragment added in changelog.d/.
  (See documentation for details)

[kanavin]

Sadly, "Fix, again, finding headers during cross compiling" broke Yocto builds, which is a fairly important cross compile use case :-)

I'm investigating, may be 'our fault'.