[ocaml-ctypes] Spinning process on osx

[Daniel Bünzli]

So I tried with libffi's head [1] and the bug still there. 

Daniel

[1] https://github.com/atgreen/libffi/commit/3dc3f32c35db5ab995a835225f6815369735ceb7

For the record that was easier than I expected. I just did 

> brew remove libffi
> brew edit libffi 
# in the url field point to the archive of a github commit
# compute the sha1 of the archive and add it to the sha1 field
> brew install libffi 
> opam remove ctypes # don't now if its needed but just in case 
> opam install ctypes