Marc-Andre Lemburg has found the culprit of the problem in r57142.

tiran · tiran · commit b5daaed30d7c · 2007-11-30T23:34:21.000Z
diff --git a/Modules/socketmodule.h b/Modules/socketmodule.h
@@ -8,7 +8,7 @@
 #   include <sys/socket.h>
 # endif
 # include <netinet/in.h>
-# if defined(__CYGWIN__) || (defined(PYOS_OS2) && defined(PYCC_VACPP))
+# if !(defined(__CYGWIN__) || (defined(PYOS_OS2) && defined(PYCC_VACPP)))
 #  include <netinet/tcp.h>
 # endif
 
