MinGW presently does not support wmain (don't have any information on whether it might in the future).
I am not sure if wmain is canon, anyway. It smells very much like a home-brewn Microsoft thing (but I may of course be wrong). Nevertheless, I've seen it around a couple of times, and it is well-known for making trouble.
The "correct" way to solve this problem as you can find on the internet is to use the CRT function __wgetmainargs from inside a normal main function. That function is neither documented, nor is it declared in the headers, so you'll have to properly declare it yourself before using it.
Personally, I have never bothered to do that (I perceive this as a hack, and I'd rather just use main), so I can't tell you an exact recipe for that.