Ignore gtk-update-icon-cache when computing build order
    
    The gtk-update-icon-cache from bootstrap should be fine to use when
    building other modules. This avoids a loop:
    
     gtk3 => at-spi2-atk => gtk2 => gtk-update-icon-cache (part of gtk3)
    
    Which can eventually be fixed at the packaging level, since at-spi2-atk
    actually only spuriously buildrequires gtk2.