configury: enable mkdtemp on _XOPEN_SOURCE incompatible hosts.

* configure.ac (POSIX_EXTRA_CPPFLAGS) <darwin*, openbsd*>: Add
-DLPOSIX_2008_COMPLIANT instead of -D_XOPENSOURCE=700.
* ext/include/_helpers (LPOSIX_2008_COMPLIANT): When manually
defined, also ensure LPOSIX_2001_COMPLIANT is defined.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>