unison: patch 2.51.3

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.

pkgs/applications/networking/sync/unison/default.nix

@@ -1,4 +1,4 @@
-{lib, stdenv, fetchFromGitHub, ocamlPackages, fontschumachermisc, xset, makeWrapper, ncurses, gnugrep
+{lib, stdenv, fetchFromGitHub, ocamlPackages, fontschumachermisc, xset, makeWrapper, ncurses, gnugrep, fetchpatch
 , enableX11 ? true}:
 
 let inherit (ocamlPackages) ocaml lablgtk; in
@@ -17,6 +17,16 @@ stdenv.mkDerivation (rec {
   nativeBuildInputs = [ makeWrapper ];
   buildInputs = [ ocaml ncurses ];
 
+  patches = [
+    # Patch to fix build with ocaml 4.12. Remove in 2.51.4
+    # https://github.com/bcpierce00/unison/pull/481
+    (fetchpatch {
+      name = "fix-compile-with-ocaml-4.12.patch";
+      url = "https://github.com/bcpierce00/unison/commit/14b885316e0a4b41cb80fe3daef7950f88be5c8f.patch?full_index=1";
+      sha256 = "0j1rma1cwdsfql19zvzhfj2ys5c4lbhjcp6jrnck04xnckxxiy3d";
+    })
+  ];
+
   preBuild = (if enableX11 then ''
     sed -i "s|\(OCAMLOPT=.*\)$|\1 -I $(echo "${lablgtk}"/lib/ocaml/*/site-lib/lablgtk2)|" src/Makefile.OCaml
   '' else "") + ''

pkgs/top-level/all-packages.nix

@@ -27313,7 +27313,6 @@ in
   unipicker = callPackage ../applications/misc/unipicker { };
 
   unison = callPackage ../applications/networking/sync/unison {
-    ocamlPackages = ocaml-ng.ocamlPackages_4_09;
     enableX11 = config.unison.enableX11 or true;
   };