gitflow: tweak for darwin

On Darwin, wrap the git-flow binary and set:

    FLAGS_GETOPT_CMD=${getopt}/bin/getopt

Without this change, I was getting the following error:

    flags:ERROR short flag required for (showcommands) on this platform

pkgs/applications/version-management/git-and-tools/gitflow/default.nix

@@ -1,4 +1,6 @@
-{ stdenv, fetchFromGitHub }:
+{ pkgs, stdenv, fetchFromGitHub }:
+
+with pkgs.lib;
 
 stdenv.mkDerivation rec {
   pname = "gitflow";
@@ -12,10 +14,17 @@ stdenv.mkDerivation rec {
     sha256 = "1i8bwi83qcqvi8zrkjn4mp2v8v7y11fd520wpg2jgy5hqyz34chg";
   };
 
+  buildInputs = optionals (stdenv.isDarwin) [ pkgs.makeWrapper ];
+
   preBuild = ''
     makeFlagsArray+=(prefix="$out")
   '';
 
+  postInstall = optional (stdenv.isDarwin) ''
+    wrapProgram $out/bin/git-flow \
+      --set FLAGS_GETOPT_CMD ${pkgs.getopt}/bin/getopt
+  '';
+
   meta = with stdenv.lib; {
     homepage = https://github.com/petervanderdoes/gitflow;
     description = "Extend git with the Gitflow branching model";