Fix RPATH when CMAKE_INSTALL_LIBDIR != lib.
Fix RPATH when CMAKE_INSTALL_LIBDIR != lib.

In some platforms ${CMAKE_INSTALL_LIBDIR} expands to `lib64`. The libraries
are correctly installed in that directory, but the RPATH is set to point to
`$ORIGIN/../lib`, where it should be `$ORIGIN/../lib64`.
google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf