ocamlbuild: add an explicit "infer" tag for menhir (Patch by Hugo Heuzard)

This allows to prevent the --infer option from being passed to Menhir
by using the negative tag -infer.

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@15598 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02