Define _XOPEN_SOURCE in configure and Python.h.

This gets compilation of posixmodule.c to succeed on Tru64 and does no
harm on Linux.  We may need to undefine it on some platforms, but
let's wait and see.

Martin says:

> I think it is generally the right thing to define _XOPEN_SOURCE on
> Unix, providing a negative list of systems that cannot support this
> setting (or preferably solving whatever problems remain).
>
> I'd put an (unconditional) AC_DEFINE into configure.in early on; it
> *should* go into confdefs.h as configure proceeds, and thus be active
> when other tests are performed.

Include/Python.h

@@ -9,13 +9,8 @@
 # define _GNU_SOURCE	1
 #endif
 
-/* Forcing SUSv2 compatibility still produces problems on some
-   platforms, True64 and SGI IRIX being two of them, so for now the
-   define is switched off. */
-#if 0
 #ifndef _XOPEN_SOURCE
-# define _XOPEN_SOURCE	500
-#endif
+# define _XOPEN_SOURCE 500
 #endif
 
 /* Include nearly all Python header files */

configure

@@ -1,5 +1,5 @@
 #! /bin/sh
-# From configure.in Revision: 1.329 .
+# From configure.in Revision: 1.330 .
 # Guess values for system-dependent variables and create Makefiles.
 # Generated by GNU Autoconf 2.53.
 #
@@ -1225,6 +1225,11 @@ VERSION=2.3
 
 SOVERSION=1.0
 
+cat >>confdefs.h <<\_ACEOF
+#define _XOPEN_SOURCE 500
+_ACEOF
+
+
 # Arguments passed to configure.
 
 CONFIG_ARGS="$ac_configure_args"

configure.in

@@ -20,6 +20,8 @@ VERSION=2.3
 AC_SUBST(SOVERSION)
 SOVERSION=1.0
 
+AC_DEFINE(_XOPEN_SOURCE, 500)
+
 # Arguments passed to configure.
 AC_SUBST(CONFIG_ARGS)
 CONFIG_ARGS="$ac_configure_args"