(svn r10353) -Fix/Feature [FS#669]: disallow (in the GUI) the building of infrastructure you do not have available vehicles for. This means that the airport building button is disabled till you can actually build aircraft. The game itself will not disallow you to build the infrastructure and this "new" behaviour can be overriden with a patch setting.

src/vehicle.h

@@ -77,6 +77,7 @@ enum VehicleType {
 	VEH_END,
 	VEH_INVALID = 0xFF,
 };
+DECLARE_POSTFIX_INCREMENT(VehicleType);
 template <> struct EnumPropsT<VehicleType> : MakeEnumPropsT<VehicleType, byte, VEH_TRAIN, VEH_END, VEH_INVALID> {};
 typedef TinyEnumT<VehicleType> VehicleTypeByte;
 
@@ -569,6 +570,7 @@ void VehicleEnterDepot(Vehicle *v);
 void InvalidateAutoreplaceWindow(EngineID e);
 
 CommandCost MaybeReplaceVehicle(Vehicle *v, bool check, bool display_costs);
+bool CanBuildVehicleInfrastructure(VehicleType type);
 
 /* Flags to add to p2 for goto depot commands */
 /* Note: bits 8-10 are used for VLW flags */