Index: src/OFDatagramSocket.m
==================================================================
--- src/OFDatagramSocket.m
+++ src/OFDatagramSocket.m
@@ -13,11 +13,13 @@
  * file.
  */
 
 #include "config.h"
 
-#define _XOPEN_SOURCE_EXTENDED
+#ifndef _XOPEN_SOURCE_EXTENDED
+# define _XOPEN_SOURCE_EXTENDED
+#endif
 
 #include <errno.h>
 
 #ifdef HAVE_FCNTL_H
 # include <fcntl.h>

Index: src/OFSequencedPacketSocket.m
==================================================================
--- src/OFSequencedPacketSocket.m
+++ src/OFSequencedPacketSocket.m
@@ -13,11 +13,13 @@
  * file.
  */
 
 #include "config.h"
 
-#define _XOPEN_SOURCE_EXTENDED
+#ifndef _XOPEN_SOURCE_EXTENDED
+# define _XOPEN_SOURCE_EXTENDED
+#endif
 
 #include <assert.h>
 #include <errno.h>
 
 #ifdef HAVE_FCNTL_H

Index: src/OFStreamSocket.m
==================================================================
--- src/OFStreamSocket.m
+++ src/OFStreamSocket.m
@@ -13,11 +13,13 @@
  * file.
  */
 
 #include "config.h"
 
-#define _XOPEN_SOURCE_EXTENDED
+#ifndef _XOPEN_SOURCE_EXTENDED
+# define _XOPEN_SOURCE_EXTENDED
+#endif
 #define __NO_EXT_QNX
 
 #include <assert.h>
 #include <errno.h>
 #include <string.h>

Index: src/OFTCPSocket.m
==================================================================
--- src/OFTCPSocket.m
+++ src/OFTCPSocket.m
@@ -13,11 +13,13 @@
  * file.
  */
 
 #include "config.h"
 
-#define _XOPEN_SOURCE_EXTENDED
+#ifndef _XOPEN_SOURCE_EXTENDED
+# define _XOPEN_SOURCE_EXTENDED
+#endif
 #define __NO_EXT_QNX
 
 #include <errno.h>
 #include <stdio.h>
 #include <stdlib.h>

Index: src/socket.m
==================================================================
--- src/socket.m
+++ src/socket.m
@@ -13,11 +13,13 @@
  * file.
  */
 
 #include "config.h"
 
-#define _XOPEN_SOURCE_EXTENDED
+#ifndef _XOPEN_SOURCE_EXTENDED
+# define _XOPEN_SOURCE_EXTENDED
+#endif
 
 #ifdef OF_NINTENDO_3DS
 # include <malloc.h>  /* For memalign() */
 #endif