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.