To generate the icon cache files. We want to avoid a dependency loop if possible; additionally, on some Debian-based systems gtk-update-icon-cache maps to the GTK2 version of the utility and the GTK3 version is renamed to gtk-update-icon-cache-3.0. To avoid a build dependency on GTK2, use the binary that we just built in-tree. https://bugzilla.gnome.org/show_bug.cgi?id=749593