Don't define _XOPEN_SOURCE on OpenBSD 2.x and 3.[012].

configure

@@ -1,5 +1,5 @@
 #! /bin/sh
-# From configure.in Revision: 1.364 .
+# From configure.in Revision: 1.365 .
 # Guess values for system-dependent variables and create Makefiles.
 # Generated by GNU Autoconf 2.53 for python 2.3.
 #
@@ -1254,10 +1254,8 @@ _ACEOF
 # of _XOPEN_SOURCE. So define it only conditionally.
 
 
-cat >>confdefs.h <<\_ACEOF
-#define _XOPEN_SOURCE 500
-_ACEOF
 
+define_xopen_source=yes
 
 # On Tru64 Unix 4.0F, defining _XOPEN_SOURCE also requires definition
 # of _XOPEN_SOURCE_EXTENDED and _POSIX_C_SOURCE, or else several APIs
@@ -1347,6 +1345,30 @@ then
 	esac
 fi
 
+# Some systems cannot stand _XOPEN_SOURCE being defined at all; they
+# disable features if it is defined, without any means to access these
+# features as extensions. For these systems, we skip the definition of
+# _XOPEN_SOURCE. Before adding a system to the list to gain access to
+# some feature, make sure there is no alternative way to access this
+# feature. Also, when using wildcards, make sure you have verified the
+# need for not defining _XOPEN_SOURCE on all systems matching the
+# wildcard, and that the wildcard does not include future systems
+# (which may remove their limitations).
+case $ac_sys_system/$ac_sys_release in
+  # On OpenBSD, select(2) is not available if _XOPEN_SOURCE is defined,
+  # even though select is a POSIX function. Reported by J. Ribbens.
+  OpenBSD/2.* | OpenBSD/3.[012])
+    define_xopen_source=no;;
+esac
+
+if test $define_xopen_source = yes
+then
+  cat >>confdefs.h <<\_ACEOF
+#define _XOPEN_SOURCE 500
+_ACEOF
+
+fi
+
 #
 # SGI compilers allow the specification of the both the ABI and the
 # ISA on the command line.  Depending on the values of these switches,

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 */