[ocaml-ctypes] Spinning process on osx

[Jeremy Yallop]

Ok, so I finally got a Mac set up to take a look at this, and tracked
down the source of the problem.  The following commit appears to fix
both the SDL and mach_absolute_time examples:

   https://github.com/yallop/ocaml-ctypes/commit/df51a59356

You can try it out using my opam-repository fork:

  opam remote add yallop https://github.com/yallop/opam-repository.git
  opam install ctypes.0.2.3 tsdl

Let me know if this fixes things, and I'll push 0.2.3 to the main
opam-repository.

Thanks for your patience, and for the precise and detailed bug report!