Don't define _XOPEN_SOURCE on OpenBSD 2.x and 3.[012].
diff --git a/pyconfig.h.in b/pyconfig.h.in
index c09a3ed..a8bede8 100644
--- a/pyconfig.h.in
+++ b/pyconfig.h.in
@@ -799,7 +799,7 @@
 
 /* Define on UNIX to activate XPG/5 features.  */
 #ifndef _XOPEN_SOURCE
-# define _XOPEN_SOURCE 500
+# undef _XOPEN_SOURCE
 #endif
 
 /* Define to activate Unix95-and-earlier features */