Fix building mumble without ice.

pkgs/applications/networking/mumble/default.nix

@@ -37,6 +37,7 @@ let
       "CONFIG+=no-bundled-speex"
     ] ++ optional (!speechdSupport) "CONFIG+=no-speechd"
       ++ optional jackSupport "CONFIG+=no-oss CONFIG+=no-alsa CONFIG+=jackaudio"
+      ++ optional (!iceSupport) "CONFIG+=no-ice"
       ++ (overrides.configureFlags or [ ]);
 
     preConfigure = ''

pkgs/top-level/all-packages.nix

@@ -14343,7 +14343,7 @@ in
       avahi = avahi.override {
         withLibdnssdCompat = true;
       };
-      qt5 = qt55; # Mumble is not compatible with qt55 yet
+      qt5 = qt55; # Mumble doesn't work with newer Qt versions.
       jackSupport = config.mumble.jackSupport or false;
       speechdSupport = config.mumble.speechdSupport or false;
       pulseSupport = config.pulseaudio or false;