I agree with Jacob Vlijm. You must post a comment addressing whoever gave you the 'Comment-answer' asking them to post it as answer separately.(Better tell them that you will surely accept the answer and up vote it(and do so without fail), if they post it as an answer. Probably that will surely work! ;-D )
But then, if you commented so and even then the user who gave the comment-answer won't post it as an answer, then you must post the answer yourself. But while doing this make sure to be morally right and make it a Community Wiki. To do this while posting an answer tick the community wiki check box as shown below :
to make an answer a Community Wiki.
For example I had a similar issue(though the question was not mine in this case) here. Make sure to read the comments there. The OP said he will post the answer but it turns out he didn't even turn up after that. So I thought the community should know that the issue is solved and so I posted the solution there(though there is no way except for the OP to accept that answer), as a Community Wiki.