qt5: explicitly disable d-bus if no '--with-d-bus'

Otherwise, the QtDBus framework will be built anyway and will try to
dynamically load `libdbus-1` at runtime. This doesn't seem desirable and
chances are low that this library will appear in a standard location.

Formula/qt5.rb

@@ -117,6 +117,8 @@ class Qt5 < Formula
       args << "-L#{dbus_opt}/lib"
       args << "-ldbus-1"
       args << "-dbus-linked"
+    else
+      args << "-no-dbus"
     end
 
     if build.with? "oci"