Added libxfce4util check
In commit f57ad643 was added line #include <libxfce4util/libxfce4util.h>. This header is also dependency from libxfce4ui, but libxfce4ui is optional for Orage. In such cases when OS does not have XFCE installed (like pure Ubuntu), and not checked libxfce4util presence, fails building Orage. Required package or header is not detected by configure script.