gtk3: build with broadway backend

The GDK Broadway backend provides support for displaying GTK+
applications in a web browser, using HTML5 and web sockets.
https://developer.gnome.org/gtk3/stable/gtk-broadway.html

pkgs/development/libraries/gtk/3.x.nix

@@ -43,6 +43,7 @@
 , cups ? null
 , AppKit
 , Cocoa
+, broadwaySupport ? true
 }:
 
 assert cupsSupport -> cups != null;
@@ -94,6 +95,7 @@ stdenv.mkDerivation rec {
     "-Dgtk_doc=${boolToString withGtkDoc}"
     "-Dtests=false"
     "-Dtracker3=${boolToString trackerSupport}"
+    "-Dbroadway_backend=${boolToString broadwaySupport}"
   ];
 
   # These are the defines that'd you'd get with --enable-debug=minimum (default).