OCamlbuild 0.15.0

OCamlbuild 0.15.0 comes with first class support for native Windows ports of OCaml
(MinGW64-w64 and MSVC). This support was historically provided by
opam-repository-mingw.

• Misc: restore CI (unix, macos, windows)
  (#328, #329, #336, #349 by Hugo Heuzard)

• Remove degraded mode for windows
  (#333 by Hugo Heuzard)

• Remove light mode (#332 by Hugo Heuzard)
• Make the codebase work on windows native
  (#329, #333, #334, #338, #339, #342, #343, #344 by Hugo Heuzard)

• No longer treat empty path in PATH env variable as the current working directory
  (#339 by Hugo Heuzard)

• Emit a warning if several calls are made to
  Ocamlbuild_plugin.dispatch -- all calls before the last one are
  ignored, which may not be what users expect.
  (#30 by Gabriel Scherer, review by whitequark)
• Rename user-rebindable {LINK,COMP}FLAGS into OCB_{LINK,COMP}FLAGS
  (#303 by Gabriel Scherer)