Alas, roll back the definition of _XOPEN_SOURCE.  It breaks the tests
for the time module, because somehow configure won't define the
symbols HAVE_STRUCT_TM_TM_ZONE, HAVE_TM_ZONE, and HAVE_TZNAME in this
case.

I've got no time to research this further, so I leave it in Jeremy and
Martin's capable hands to find a different solution for True64 (or to
devise a way to get the time tests to succeed while defining
_XOPEN_SOURCE).
diff --git a/configure b/configure
index 007ae0f..4bd4740 100755
--- a/configure
+++ b/configure
@@ -1,5 +1,5 @@
 #! /bin/sh
-# From configure.in Revision: 1.332 .
+# From configure.in Revision: 1.333 .
 # Guess values for system-dependent variables and create Makefiles.
 # Generated by GNU Autoconf 2.53.
 #
@@ -1225,12 +1225,6 @@
 
 SOVERSION=1.0
 
-
-cat >>confdefs.h <<\_ACEOF
-#define _XOPEN_SOURCE 500
-_ACEOF
-
-
 # Arguments passed to configure.
 
 CONFIG_ARGS="$ac_configure_args"