* ocaml-modules/dtoa: disable hardening (strictoverflow) on darwin
This prevents a compile-time error on macOS:
clang-4.0: error: argument unused during compilation: '-fno-strict-overflow'
[-Werror,-Wunused-command-line-argument]
* ocaml-modules/dtoa: remove unnecessary createFindLibDestdir
* ocaml-modules/dtoa: remove ounit from build inputs