void gtk_window_set_deletable ( GtkWindow* window, gboolean setting )
By default, windows have a close button in the window frame. Some
[window managers][gtk-X11-arch] allow GTK+ to
disable this button. If you set the deletable property to
using this function, GTK+ will do its best to convince the window
manager not to show a close button. Depending on the system, this
function may not have any effect when called on a window that is
already visible, so you should call it before calling gtk_widget_show().
On Windows, this function always works, since there’s no window manager policy involved.