set_spacing() sets the spacing between the buttons, in pixels. See also gtk::hbutton_box_set_spacing_default() or gtk::vbutton_box_set_spacing_default() to set the spacing default across an entire application. If you don't change it, the default setting is currently 10 pixels for GtkVButtonBox, or 30 for GtkHButtonBox.
Note that this method is deprecated in GTK+ 2.0. Where forward compatibility is an issue, use set_layout() instead.
