stdlib: mkdtemp returns an error on substandard hosts.

* ext/posix/stdlib.c (mkdtemp): Move POSIX compliance guards
inside the function definition, with an alternate function body
that returns an error when there is no mkdtemp in the C library.
(posix_stdlib_fns): Remove POSIX compliance guard from mkdtemp.

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