Deleting a already present libjuce.a solved the problem. The problem came because I already had libjuce.a in the bin folder, but not the Linux version, but the Mac version! And somehow, ar didn’t like that.
So I think it would be good if the mac and linux libs would have 2 different names, because if one’s using one single computer for os x, linux and windows, and all OS’es access the same bin folder, then this problem occurs.