OCamlbuild 0.16.1

OCamlbuild 0.16.1 improves ocamlbuild's behavior when cross-compiling for Windows.

• Set default values of Options.ext_* and Options.exe from the output of
  Options.ocamlc "-config" (Useful when Options.ocamlc is a cross compiler)
  (#353, #361 by Pierre Boutillier, reviewed by Kate Deplaix and Hugo Heuzard,
  requested by whitequark in #181)
• Misc: continue improving the CI
  (#355 by Hugo Heuzard)