configure: Enable spice-gtk when have all necessary spice packages
    
    When neither --with-spice-gtk=yes nor --with-spice-gtk=no is used,
    spice-gtk is supposed to be automatically enabled/disabled depending
    on its availability. However, this is not perfectly working as once
    spice-gtk has been detected as available, configure will fail if
    spice-protocol or spice-controller are too old. In this case, spice-gtk
    support should just be disabled rather than configure failing