Make ocaml-menhir-doc subpackage optional
    
    ocaml-odoc is required to build the menhir documentation.
    Unfortunately that depends on half the world.  As Menhir itself is
    quite an important low level package this is very inconvenient.
    
    We can make things much easier on ourselves by making this
    documentation optional (it is, after all, available online).
    Use --with odoc if you want to build the documentation.