Thanks for the suggested patch but I am not persuaded that there is a problem needing fixing on the Python side.  As I see it, Python's posixmodule builds fine in the environments we claim to support, including macOS, and has for a long time.  It appears that the build failure reported here is because Nix is not properly emulating a macOS build environment and that builds under Nix have been producing warnings prior to 3.7 so it's not a new issue.  Rather than trying to patch around the Nix problem on the Python side, and potentially introducing problems on platforms that Python "officially" supports, it seems to me the better options are for either (1) Nix to better emulate a real macOS build environment by supplying a more correct set of header files (which also might help other third-party packages trying to use Nix for macOS builds); or, if that is not possible, (2) to carry a local patch for Python when building under Nix.  I'm open to persuasion but it would have to be a very compelling argument.  We have never claimed to support macOS builds on anything other than Apple-supplied macOS development environments running on macOS and I don't see that changing.