configure: for cross, a matching ocamlrun must be on the system.
(Patch by Adrien Nader!)

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@13852 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
configure: add --target and use "target" instead of "host" for variables
(Patch by Adrien Nader!)

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@13853 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02