<% Layout.activatelink %>
render a link for activating the layout, but only if layout is *not* the currently active layout

Sourcecode in Layout/macros.js:
1:   function activatelink_macro(param) {
2:      if (this.isDefaultLayout())
3:         return;
4:      Html.link({href: this._parent.href() + "?activate=" + this.alias},
5:                param.text ? param.text : getMessage("Layout.activate"));
6:      return;
7:   }