set common paths to reuse in all CMake lib and path finders.

I just had to add /usr/lib64 to CMake/FindGLIB.cmake for it to be found CentOS/Fedora (and perhaps others) distributions. Seems like we should be able to reuse variables common path across all the various CMake finders.

I'm not sure of the best approach for this, as i don't know the various common directories across all distributions or whether path ordering is a factor.

In any case, if we do solve this, then it should make it easier to provide Windows support for these finders at some later point also.