ocamlbuild: remove the deprecated ~tags from ocaml_specific.ml, and warn on use

The parameter ~tags of Rule.rule has been ignored and deprecated for
a while, but our own code in ocaml_specific.ml was still using it --
confusing several plugin writers. This patch, inspired by a patch
proposal by Anil Madhavapeddy on PR#6059, also makes Rule.rule emit
a warning when this parameter is used. Anil originally removed it
completely from the interface, but I would like to avoid breaking the
compilation of plugin code.

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@13987 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02