There's no test currently for cygwinccompiler. Not sure how to add them.

ping on this

Can you say a little bit more about what motivated you to make this change? Did you encounter an error? How is it that this issue is only relevant now, 21 years after the module was created?

Projects supporting windows would always have a if condition to fix this, for eg:
https://github.com/h5py/h5py/blob/master/setup_build.py#L107-L108
https://github.com/msys2/MINGW-packages/blob/707dc76b8c2a11c6e896f72999efcfe68c7e1054/mingw-w64-gobject-introspection/0028-no-rpath-on-nt.patch#L8

MinGW patches python to remove rpath to make it easier to port projects
https://github.com/msys2/MINGW-packages/blob/2a99c1fe8b5c24f69585bc4836ddfaadca6c2891/mingw-w64-python/0085-distutils-compiler-enable-new-dtags.patch#L27-L29

Also see this setuptools issue,
pypa/setuptools#2753

Nvm, I need to add it to msvc compiler. Not cygwin.

Sorry about the noise

We do see a related failure in SageMath on Cygwin when using SETUPTOOLS_USE_DISTUTILS=local.
https://trac.sagemath.org/ticket/32944

PR pypa/setuptools#2909 adds a CI for Cygwin, but it does not expose this failure because apparently linking is not actually tested