Bug fix (coq_makefile): Adding unix.cma and threads.cma dependencies for...

... grammar in campl4

The calls to camlp4 generated by coq_makefile where bugged since grammar.cma depends on threads.
This should fix it. 

I had to add the option "-I $(CAMLLIB)threads/" to the loadpath of camlp4 (of course it relies on the "unix" threads to be present in ocaml distribution). It should work for windows too, but I haven't checked it.