mumble: Fix PLUGIN_PATH definition so it can find the CELT library

pkgs/applications/networking/mumble/default.nix

@@ -37,12 +37,12 @@ let
       "CONFIG+=bundled-celt"
       "CONFIG+=no-bundled-opus"
       "CONFIG+=no-bundled-speex"
+      "DEFINES+=PLUGIN_PATH=${placeholder "out"}/lib/mumble"
     ] ++ optional (!speechdSupport) "CONFIG+=no-speechd"
       ++ optional jackSupport "CONFIG+=no-oss CONFIG+=no-alsa CONFIG+=jackaudio"
       ++ (overrides.configureFlags or [ ]);
 
     preConfigure = ''
-       qmakeFlags="$qmakeFlags DEFINES+=PLUGIN_PATH=$out/lib/mumble"
        patchShebangs scripts
     '';