Good, I could make it work thanks to your combined suggestions! More
precisely:

1. rename r_lang_parser.mly to r_lang_parser_aux.mly
2. create r_lang_parser.mlypack which contents is a single line:
R_lang_parser_aux
3. add a line in the _tags file (the package(camlp4.lib) syntax did not
work for me, I used the tag pkg_camlp4.lib which is created in the oasis
ocamlbuild plugin).

So it does appear that the function to call menhir in ocamlbuild lacks an
argument. I'll make a report in mantis for that.

Thank you both!
ph.

2012/9/20 Benoit Montagu <benoit.montagu@m4x.org>

> The typical use case for mlypack is the following: you have several parser
> files (say, A.mly and B.mly) that menhir needs to combine (yes, menhir can
> do that), and you want menhir to create a parser C.ml/C.mli.
> To do so, you create C.mlypack, whose contents is:
> A
> B
>
> And then ocamlbuild should work.
>
> In your case, r_lang_parser.mlypack should contain
> Foo
>
> where Foo.mly is your menhir file.
>
> I don't know about how to pass extra arguments to menhir.
>
> -- Benoit
>