Major Description
Currently, pip itself allows a custom option to set via --install-option. However, when you pass this option to pip, it passes that option to all dependency installation. Please also see https://github.com/pypa/pip/issues/1883
It is very hacky or impossible to use this option to pass the option to a specific package. They are discussing to have a more general option.
Once they have a way to do it, we should remove the environment variables from setup.py and switch it to proper options.