iHave(structure)

// iHave(structureTypeOrID) returns true if you have
// at least one of those structures fully built
var iHave = (function(){
  var buildingIsBuilt = function(building) {
    return building.status == BUILT;
  }
  return function (structure) {
    return enumStruct(me,structure).some(buildingIsBuilt);
  }
})();

// You need a HQ before you can design units or build defence towers
if (iHave(HQ)) {
  // build defence towers
} else {
  // instruct a truck to build a HQ!
}