Also check sharing of immutable constants.

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14470 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
PR#6349: ocamlbuild.cm{o,x} must not be included in ocamlbuild.cm{a,xa}

The bug, reported by Jacques-Pascal Deplaix, was only in trunk -- it
appeared in the bootstrap->Makefile transition.

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14471 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02