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 commit v3.8.0-rc1~71^2
(cmSystemTools: use the actual case for root detection, 2017-01-18).

Issue: #16648