Index: include/ruby/win32.h
===================================================================
--- include/ruby/win32.h	(revision 20912)
+++ include/ruby/win32.h	(working copy)
@@ -27,6 +27,7 @@
 
 // #include <stdarg.h> conflict with varargs.h?
 #if !defined(WSAAPI)
+#define FD_SETSIZE 4096
 #include <winsock2.h>
 #include <ws2tcpip.h>
 #endif