Change usage of CGNS_ROOT variable in CMakeLists
The environment variable CGNS_ROOT is currently passed as PATHS to find_library and find_path in CMakeLists.txt. Consequently, a version of the library found in the default system locations has priority over one located in the path given by CGNS_ROOT.
It seems to me that the path specified by the user in CGNS_ROOT should have priority over the default system locations, which is obtained by changing PATHS to HINTS in the calls to find_library and find_path.
This merge request only deals with CGNS, but the search for PETSc libraries may suffer from the same problem.