openbox: add platforms, split version

pkgs/applications/window-managers/openbox/default.nix

···
 , imlib2, pango, libstartup_notification, makeWrapper }:
 
 stdenv.mkDerivation rec {
-  name = "openbox-3.6.1";
+  name = "openbox-${version}";
+  version = "3.6.1";
 
   buildInputs = [
     pkgconfig libxml2
···
     description = "X window manager for non-desktop embedded systems";
     homepage = http://openbox.org/;
     license = stdenv.lib.licenses.gpl2Plus;
+    platforms = stdenv.lib.platforms.linux;
   };
 }