How can I get ocamlbuild to include paths to external libraries when building documentation?

Ocamlbuild gives me errors when building documentation, when my interface files contain references to an external library. I have set up my plugin and _tags file to correctly compile the code, but cannot get the documentation to work. The command that ocamlbuild executes is [1], which gives error [2]. Module M is declared in an external library. I get no errors if I manually type [3], where ~/mylibs contains the appropriate library.

[1] ocamlfind ocamldoc -dump a.odoc a.mli
[2] Unbound module M
[3] ocamlfind ocamldoc -dump a.odoc -I ~/mylibs/ a.mli

Thank you.