MinGW: use WinSock2

After reading a bit further, it appears that ws2_32 (Windows Sockets 2)
is included in all versions of Windows and backwards compatible with
wsock32, at least according to

 http://msdn.microsoft.com/en-us/library/ms740673%28VS.85%29.aspx.

Only Win95 seems to require a manual installation; is not a big deal.

So I think we can drop this whole business of detecting 64 bit MinGW and
just use -lws2_32 for all MinGW platforms.