configure.ac: complain when xmlto cannot be found
configure.ac: complain when xmlto cannot be found

related to #338

Dettorer/i3status,Gravemind/i3status,Gravemind/i3status,Gravemind/i3status,peder2tm/i3status-netdev,Dettorer/i3status,Gravemind/i3status,Dettorer/i3status,Dettorer/i3status,i3/i3status,peder2tm/i3status-netdev,peder2tm/i3status-netdev,i3/i3status,peder2tm/i3status-netdev,i3/i3status,i3/i3status