Make CMAKE_ROOT independent of case of path used to invoke cmake

We compute the location of CMAKE_ROOT and other resources relative to the location of our own executable. On some platforms this path is computed in a way that depends on the case of the path used to invoke the executable. Convert the result to the actual case preserved by the filesystem on disk in order to make it consistent regardless of how the executable is launched.

This approach generalizes the fix made by !414 (merged).

Issue: #16648 (closed)