review OPAL CMakeModule files

Searching for a library with

FIND_LIBRARY (GSL_LIBRARY gsl
    HINTS $ENV{GSL_ROOT_DIR}/lib $ENV{GSL_LIBRARY_PATH} $ENV{GSL_LIBRARY_DIR} $ENV{GSL_PREFIX}/lib $ENV{GSL_DIR}/lib $ENV{GSL}/lib
    PATHS ENV LIBRARY_PATH
)

can fail if the library is installed in /lib or /lib64. For some unknown reasons this works on Merlin-5 but fails on Merlin-6 if e.g. libgsl is installed.

What is the problem?

On RHEL7 /usr/lib64 is a symbolic link to /lib64.
If e.g. GSL_ROOT_DIR is not set in the environment /lib and /lib64 are used to search for the library.
In RHEL7 most system libraries are installed in /lib64, which is the first hint if GSL_ROOT_DIR is not set.