Ocamlbuild directory inclusion problem

[Dario Teixeira <darioteixeira@yahoo.com>]

Hi,

I've come across another issue with Ocamlbuild, and I'm not sure if this one
is a bug or a feature.  Consider the project tree below; note that everything
under lib/ is supposed to produce a foo.cma library, while cmd/ is a command
line application that uses foo.cma (and which can be safely ignored for now).

|-- _tags
|-- lib/
|   |-- _tags
|   |-- foo.mllib
|   |-- foo_core.mlpack
|   |-- foo_core/
|   |   |-- (...)
|   |
|   |-- foo_ext.mlpack
|   |-- foo_ext/
|       |-- (...)
|
|-- cmd/
    |-- (...)

The file lib/_tags just contains "<foo_core> or <foo_ext>: include".  If I issue
"ocamlbuild foo.cma" inside the lib/ directory, then foo.cma gets built as expected.
However, if I move to the parent directory and issue "ocamlbuild lib/foo.cma",
I get an error:

ocamlc -pack lib/foo_ext/beta.cmo lib/foo_ext/alpha.cmo -o lib/foo_ext.cmo
File "_none_", line 1, characters 0-1:
Error: The implementation (obtained by packing)
       does not match the interface lib/foo_ext.mli:
       Modules do not match:
         sig val num : int end
       is not included in
         Foo_core.Zero.S
       Unbound module type Foo_core.Zero.S
Command exited with code 2.


The problem is of course that "-I lib" should be added to the invocation of ocamlc.
For this purpose I've created a myocamlbuild.ml in the root directory, with the
following contents:

let () = dispatch begin function
        | After_rules ->
                Pathname.define_context "lib/foo_ext" ["lib"];
        | _ -> ()
end


However, this doesn't work as expected.  Perhaps I've misunderstood the purpose
of Pathname.define_context?  In any case, I would expect a simple example like
this one to work even without the need for a myocamlbuild.ml plugin.  So, what
is missing here?

Thanks in advance!
Best regards,
Dario Teixeira