Index: src/OFStreamObserver.h
==================================================================
--- src/OFStreamObserver.h
+++ src/OFStreamObserver.h
@@ -14,11 +14,13 @@
 #endif
 
 #import "OFObject.h"
 
 #ifdef _WIN32
-# define _WIN32_WINNT 0x0501
+# ifndef _WIN32_WINNT
+#  define _WIN32_WINNT 0x0501
+# endif
 # include <windows.h>
 #endif
 
 @class OFStream;
 #ifdef OF_HAVE_POLL

Index: src/OFStreamSocket.h
==================================================================
--- src/OFStreamSocket.h
+++ src/OFStreamSocket.h
@@ -10,11 +10,13 @@
  */
 
 #import "OFStream.h"
 
 #ifdef _WIN32
-# define _WIN32_WINNT 0x0501
+# ifndef _WIN32_WINNT
+#  define _WIN32_WINNT 0x0501
+# endif
 # include <winsock2.h>
 #endif
 
 /**
  * \brief A class which provides functions to create and use stream sockets.