[ocaml-ctypes] Undefined symbols error on OS X

[Travis]

I'm attempting experimental OCaml bindings to Vowpal Wabbit and am stuck
with the error here:
https://gist.github.com/travisbrady/21a83ac57790455d4b9d545e8b18539a

My code is visible at: https://github.com/travisbrady/ocaml-vw

This is with OCaml 4.03.0, ctypes 0.11.3, Darwin 14.5.0 and libffi 3.0.13
installed via Homebrew.

After changing the package order manually I still get the error. Is it an
issue with my code? Should I  install a more recent version of libffi?

thank you
Travis
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