I have just been experimenting with SEND_ERROR, and am willing to commit this. My preference is to have the config process to stop at the point the dependency is not met such that a developer does not have to search config logs/messages to find out just why the config did not work (but I take your point about being able to check other config options). This approach is also identical to that used for gthread inclusion a few lines down. If SEND_ERROR is used for bad gtk dependency, perhaps we should make this the same for gthread? (for a consistent approach).
So to summarise, I'm happy to change this approach if you think this would be preferred by users, but I think we should make this approach consistent with gthread at the same time.