To cut a long story short, it didn't worked. The build of glib and libnice isn't compatible with the MinGW Qt5 build. Compiling Voptop with it works fine, but it crashes immediately in process start time.
I will not spend more time in porting a newer libnice version to Windows, because I am not really satisfied with libnice at all. To be honest, I think it is a bad piece of code. I just used it to save time because making my own STUN/TURN implementation is serious work. I think it is time to tackle this problem and do the work.
The next version of Voptop will be released with Socks 5 proxy support still based on the libnice, in this way it works at minimum for Linux and MacOS X and the Windows user will have to wait until I made my own STUN/TURN implementation.