Pass -thread to C files as well
Pass -thread to C files as well

ocamlbuild>=4.03 passes "-package" to ocamlfind for C files
(PR6794). This break packages with C bindings using threads (PR#6809).

gerdstolpmann/oasis,Chris00/oasis,Chris00/oasis,Chris00/oasis,gerdstolpmann/oasis,gerdstolpmann/oasis