gtk3: add updateScript

pkgs/development/libraries/gtk+/3.x.nix

@@ -12,15 +12,13 @@ assert cupsSupport -> cups != null;
 with stdenv.lib;
 
 let
-  ver_maj = "3.22";
-  ver_min = "26";
-  version = "${ver_maj}.${ver_min}";
+  version = "3.22.26";
 in
 stdenv.mkDerivation rec {
   name = "gtk+3-${version}";
 
   src = fetchurl {
-    url = "mirror://gnome/sources/gtk+/${ver_maj}/gtk+-${version}.tar.xz";
+    url = "mirror://gnome/sources/gtk+/${gnome3.versionBranch version}/gtk+-${version}.tar.xz";
     sha256 = "61eef0d320e541976e2dfe445729f12b5ade53050ee9de6184235cb60cd4b967";
   };
 
@@ -76,6 +74,13 @@ stdenv.mkDerivation rec {
     moveToOutput bin/gtk-launch "$out"
   '';
 
+  passthru = {
+    updateScript = gnome3.updateScript {
+      packageName = "gtk+";
+      attrPath = "gtk3";
+    };
+  };
+
   meta = with stdenv.lib; {
     description = "A multi-platform toolkit for creating graphical user interfaces";