ocaml-modules/dtoa: disable hardening (strictoverflow) on darwin (#33443)

* 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