For people who are still using the X11-based components of Radiance, a new version of XQuartz supports the Apple M1 Macs, available at XQuartz - Releases.
That is a good alternative for those who do not want to use a package manager such as macports or homebrew. Not sure though if this provides all necessary libraries to build Radiance from sources?
Best, Lars.
My system has /opt/X11, which seems to contain the necessary files, but I am not sure where it came from and in any event is not an M1.