PR#6495: add a safe_string tag to ocamlbuild (patch by Anil Madhavapeddy)
    
    It kicks in for interface inference and compiler invocations.
    safe_string does also alter the output of inferred types if
    short_paths is used (since "bytes" is shorter than "string"),
    but this requires a change to the ocamldoc driver and isn't
    addressed in this PR.
    
    git-svn-id: http://caml.inria.fr/svn/ocaml/version/4.02@15049 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02