When compiling an extension like

ext_modules = [
    Extension(
        "_foobar",
        [
           # ...
        ],
        language="C++",
    )
]

setuptools appears to use the CC environment variable when compiling the project. E.g.,

CC=clang++ pip3 install . --user --verbose

will switch to clang++. Since this is a C++ project (as specified in the extension), I would have expected CXX to have an effect, and not CC.