Follow up r2123, make PREFIX work again if LIBDIR isn't specified.
Follow up r2123, make PREFIX work again if LIBDIR isn't specified.

* SConstruct:
  (default_libdir): Use $PREFIX as default prefix dir.
  (PathVariable LIBDIR): Fix typo.

jandre/serf,jandre/serf,jandre/serf,jandre/serf