[CMake] how to (portably) change file permissions -- besides INSTALL?

[kent williams]

Subject line says it all.  I could explain what I'm trying to do but
it would be a case of "Too long, didn't read" for most of you.

You can change permissions with the CMake 'install' command, the
'file(COPY)' and 'file(INSTALL)' command but I can't find the command
that changes the permissions of an existing file.  Unless I'm missing
something, which I almost always am.