[pfff] findlib changes

Summary:
I've been trying to use pfff with ocamlfind, and these are the changes
I need to make to get things working :)

Test Plan:
make uninstall-findlib
make install-findlib
Build my awesome project!

Reviewers: pad, jwatzman

Reviewed By: pad

Differential Revision: https://phabricator.fb.com/D1214968

Task ID: 3777352