PR#6058: have ocamlbuild add -thread for all "link" tags

The -thread option is only needed when creating a compilation unit
("compile" tag handled separated), or linking libraries into a final
program ("link";"program"). However, ocamlfind will fail with an error
if neither of -thread or -vmthread is passed into the command-line of
any linking step, such as when creating a cm(x)a archive. The present
fix enables the -thread option for all linking steps, which should fix
any -use-ocamlfind issue and be harmless in other cases.


git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@13934 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02