CMakeLists.txt: use CMAKE_INSTALL_FULL_LIBDIR for runpath on darwin

Without the change on systems where `CMAKE_INSTALL_LIBDIR` is an
absolute path outside `CMAKE_INSTALL_PREFIX` (like `nixpkgs`) libraries
ended up embedding wrong RPATH and libraries failed to load.

The change uses suggestion from https://cmake.org/cmake/help/latest/module/GNUInstallDirs.html
to use `CMAKE_INSTALL_FULL_LIBDIR` (similar to library install code)
to enbed it as an RPATH.