cups: remove xdg_utils dependency

`cups.desktop` that depends on some fixed version of `xdg-open` is not
particularly useful; it should use `xdg-open` from the environment
it's being run from.

As a side effect, one can now fiddle with `xdg_utils` package without
rebuilding pretty much every single one of graphical packages (they
all depend on `cups` through their graphical toolkits).

pkgs/misc/cups/default.nix

@@ -1,5 +1,5 @@
 { stdenv, fetchurl, pkgconfig, zlib, libjpeg, libpng, libtiff, pam
-, dbus, acl, gmp, xdg_utils
+, dbus, acl, gmp
 , libusb ? null, gnutls ? null, avahi ? null, libpaper ? null
 }:
 
@@ -17,7 +17,7 @@ stdenv.mkDerivation {
   };
 
   buildInputs = [ pkgconfig zlib libjpeg libpng libtiff libusb gnutls avahi libpaper ]
-    ++ optionals stdenv.isLinux [ pam dbus.libs acl xdg_utils ] ;
+    ++ optionals stdenv.isLinux [ pam dbus.libs acl ];
 
   propagatedBuildInputs = [ gmp ];
 
@@ -54,8 +54,7 @@ stdenv.mkDerivation {
       "CUPS_PRIMARY_SYSTEM_GROUP=root"
     ];
 
-  postInstall =
+  postInstall = ''
-    ''
       # Delete obsolete stuff that conflicts with cups-filters.
       rm -rf $out/share/cups/banners $out/share/cups/data/testprint
 
@@ -71,6 +70,10 @@ stdenv.mkDerivation {
           mv "$f" "''${f/org\.cups\./}"
         fi
       done
+    '' + optionalString stdenv.isLinux ''
+      # Use xdg-open when on Linux
+      substituteInPlace $out/share/applications/cups.desktop \
+        --replace "Exec=htmlview" "Exec=xdg-open"
     '';
 
   meta = {