unistd: stub gethostid for POSIX implementations that dropped it.

* configure.ac (AC_CHECK_FUNCS): Add gethostid.
* ext/posix/unistd.c (Pgethostid): If there is no gethostid on
this host, return an error rather than trying to call it.

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