Remove dependency to which (not available everywhere)
Remove dependency to which (not available everywhere)

ocamllabs/ocaml-ctypes,yallop/ocaml-ctypes,dsheets/ocaml-ctypes,whitequark/ocaml-ctypes,dsheets/ocaml-ctypes,dsheets/ocaml-ctypes,ocamllabs/ocaml-ctypes,whitequark/ocaml-ctypes,yallop/ocaml-ctypes,whitequark/ocaml-ctypes