Simply, run it for the subclasses for `UnixCCompiler`

use `os.sep` instead of hardcoding `\\`
also, fix appropriate tests

it depends on pydantic-core which requires rust to work
also, takes a few minutes to build.

…rted

distutils currently doesn't support pytest collection that doesn't
start at least at the distutils dir or above (and not distutils/tests)
since it requires the local distutils being imported before the tests are run,
otherwise the stdlib distutils takes precedence.

Adjust the pytest call to not pass a path to work around this.

Since pytest currently fails to skip collecting venvs with mingw python
(see pytest-dev/pytest#12544) move the venv
to /tmp instead.

MSYS2 has stopped installing gcc compatibility binaries in clang environments
by default some time ago, and distutils is currently hardcoded to look for "gcc",
while only cc/c++ and clang/clang++ are in PATH.

Work around for now by explicitely setting CC/CXX to override the defaults.

Idealy distutils would try to look harder for a valid compiler before giving up,
but this can be improved in the future.