That's because there are duplicate #defines, and when Winsock2 is defined
before windows.h then windows.h detects that and prevent redefinition.

See
http://social.msdn.microsoft.com/Forums/windowsdesktop/en-US/4a90b143-1fb8-43e9-a54c-956127e0c579/windowsh-and-winsock2h?forum=windowssdk

[JES: fixed commit message]
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>