chromium: Use system libvpx for version 25.

Unfortunately, we have build errors for version 25 in the bundled libvpx:

http://hydra.nixos.org/build/4173075
http://hydra.nixos.org/build/4173066

As I can't reproduce this on my local system (I've disabled the option
CONFIG_CC_STACKPROTECTOR here), let's just hope that libvpx is the only part
that fails during build because of this.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>

pkgs/applications/networking/browsers/chromium/default.nix

@@ -14,6 +14,9 @@
 , glib, gtk, dbus_glib
 , libXScrnSaver, libXcursor, mesa
 
+# dependencies for v25
+, libvpx
+
 # dependencies for >= v26
 , protobuf, speechd, libXdamage
 
@@ -70,6 +73,8 @@ let
     use_system_skia = false;
     use_system_sqlite = false; # http://crbug.com/22208
     use_system_v8 = false;
+  } // optionalAttrs (post24 && !post25) {
+    use_system_libvpx = true;
   };
 
   defaultDependencies = [
@@ -115,6 +120,7 @@ in stdenv.mkDerivation rec {
     ++ optional cupsSupport libgcrypt
     ++ optional pulseSupport pulseaudio
     ++ optional post24 pciutils
+    ++ optional (post24 && !post25) libvpx
     ++ optionals post25 [ protobuf speechd libXdamage ];
 
   opensslPatches = optional useOpenSSL openssl.patches;