ocamlbuild: make -plugin-tags and -use-ocamlfind interact better
    
    When both those features are used together, we use ocamlfind packages
    instead of hardlinking unix.cmxa and ocamlbuildlib.cmxa during the
    ocamlbuild plugin compilation. This helps to avoid double-linking
    errors (with unix.cmxa and "-package unix") when -plugin-tags request
    ocamlfind packages depending on unix (or ocamlbuild).
    
    This change was designed and implemented in collaboration with Jun Maillard.
    
    git-svn-id: http://caml.inria.fr/svn/ocaml/version/4.01@14068 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02