Update to support 3.20's ExternalProject
CMake 3.20 has had a lot of work done by @craig.scott to improve internal structures. The superbuild currently has tendrils expecting these things to be where they always have been. I suspect many of these have been upset and needs to be put back into place.
@utkarsh.ayachit I'll need time to do this upgrade at some point, preferrably before 3.20.0 is out (the freeze is 10 Mar, so 10 Apr or so would be the final release).