* "passing argument 1 of ‘gtk_message_dialog_format_secondary_markup’ from
  incompatible pointer type"
  * dialog was in fact constructed as GtkMessageDialog using
    gtk_message_dialog_new_with_markup(), so use it as such
* "passing argument 1 of ‘gtk_widget_destroy’ from incompatible pointer type"
  * use GTK_WIDGET() on dialog GtkMessageDialog*
* "passing argument 1 of ‘g_object_get_data’ from incompatible pointer type"
  * use G_OBJECT on dialog GtkWidget*
* "passing argument 1 of ‘gtk_dialog_set_response_sensitive’ from incompatible
  pointer type"
  * use GTK_DIALOG on dialog GtkWidget*
* "passing argument 1 of ‘gtk_message_dialog_set_image’ from incompatible
  pointer type" and "passing argument 1 of ‘gtk_dialog_add_buttons’ from
  incompatible pointer type"
  * use GTK_MESSAGE_DIALOG() on dialog GtkWidget*
* "passing argument 1 of ‘g_object_set_data_full’ from incompatible pointer
  type"
  * use G_OBJECT() on dialog GtkWidget*:x
* "passing argument 3 of ‘g_signal_connect_data’ from incompatible pointer
  type"
  * use G_CALLBACK() on dialog_response_cb
* "passing argument 1 of ‘gtk_window_set_skip_taskbar_hint’ from incompatible
  pointer type"
  * use GTK_WINDOW() on dialog GtkWidget*

How awful C is :-/  GtkMessageDialog is a GtkDialog is a GtkWindow is
a GtkWidget is a GObject, but still ...