Fix the creation of libcvmfs_cache.a on macOS
Fix the creation of libcvmfs_cache.a on macOS

On macOS (maybe also on Linux?), the libtool command can be used
instead of the "combine_libs" script to create a stand-alone
libcvmfs_cache.a library.

DrDaveD/cvmfs,cvmfs/cvmfs,DrDaveD/cvmfs,cvmfs/cvmfs,cvmfs/cvmfs,DrDaveD/cvmfs,DrDaveD/cvmfs,cvmfs/cvmfs,cvmfs/cvmfs,cvmfs/cvmfs,DrDaveD/cvmfs,DrDaveD/cvmfs,DrDaveD/cvmfs,cvmfs/cvmfs