Add the system CFFI-LIBFFI to ASDF dependencies.
Move OS X library loading paths from the :unix spec to :darwin.
Revert "Add the system CFFI-LIBFFI to ASDF dependencies."

This reverts commit 2bf722c0748076a954c57e2d30edc90067653d52.
Merge pull request #1 from tuscland/master

Minor changes to support LispWorks on OS X