Why CFLAGS not working with pip in Windows

Question

This is about Windows! I have a modern LLVM setup to work with distutils via compiler=mingw32 option in config file. But newer clang versions are more strict with warnings, so I want to pass -Wno-error=int-conversion via CFLAGS to install multidict in python 3.12. so I kindly use:

$env:CFLAGS="-Wno-error=int-conversion"
pip install multidict

But it fails and clearly the gcc command do not include CFLAGS. It looks like distutils/setuptools bug. Any alternative to workaround this?