Alas, roll back the definition of _XOPEN_SOURCE. It breaks the tests

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).