devel/cmake: Update to 3.20.1

This new version of CMake is much more picky with the language
than previous versions, so there was a fair bit of fallout --
patched before this push with plenty of "fix in advance of CMake
3.20" commits.

Release notes are at
https://cmake.org/cmake/help/v3.20/release/3.20.html

Thanks to antoine@ for multiple exp-runs.