ocamlbuild output-obj

[Anil Madhavapeddy <anil@xxxxxxxxxx>]

I've been putting in fixes into 4.1 for ocamlbuild, and this simple patch adds 
support for output-obj:

http://caml.inria.fr/mantis/view.php?id=6059

It should be all we need to compile up Xen kernels, now that all the other 
libraries use ocamlfind.

-anil