Add patch which fixes builds with (at least) ocaml 4.12.0, and
remove build constraint for 4.09. Necessary because unison built
with 4.09 is not compatible with unison built with 4.12, e.g. on
recent Homebrew.
(cherry picked from commit 7282b4fc0389217c60fe788cb80d10f3e80e447d)