Your best bet will be to talk to the people who already know libpurple. There may be a good reason why instructions to build it with Visual Studio aren't included (i.e. a reason other than cost or it being from MS). Maybe the code will need non-trivial modifications to work in VS. But this may already have been done; doesn't Pidgin use libpurple, and I know that works on Windows.
But if you just want to do it yourself and find out what needs doing, best way to start is by looking at the error and the code it comes from and trying to determine what is causing the error. Is there a missing bracket before a *? What exactly is the code that fails? The next error complains about socklen_t: again, what is the code; the error suggests it could be trying to use something that hasn't been defined yet. Googling socklen_t finds it's part of the Unix header sys/socket.h - the way sockets work on Unix and Windows have significant differences (e.g. lack of OOB support on Windows) and you may have to modify libpurple substantially so that it can work with both implementations.