The change in commit v3.11.3~3^2 (cmSystemTools: Revert GetRealPath
implementation on Windows, 2018-05-29) broke the feature added by commit
v3.11.0-rc1~64^2 (Windows: Add support for running CMake tools through a
symlink, 2018-01-21).  Use `GetRealPathResolvingWindowsSubst` in the
latter code path to restore it.

Fixes: #17882