There is absolutely no need for a feature that allows you to mark a comment as an answer. In fact, that goes against one of the primary tenants of the site: questions and answers are the primary focus, while comments are just "noise".
The better solution is to simply leave an
@username reply for that person, informing them that their comment actually answered your question and requesting that they post it as an answer so that you can accept it. I've seen this done numerous times (and had people do it with me), and my experience has been that users are more than happy to oblige.
If (and only if) they don't come back and post their comment as an answer within a reasonable frame of time (say, a few days) should you consider posting your own answer to the question, as Mike suggests.