AC_XENIX_DIR: Rewrite using AC_CANONICAL_HOST.

AC_XENIX_DIR: Rewrite using AC_CANONICAL_HOST.

AC_XENIX_DIR is an obsolete macro, defined as AC_HEADER_DIRENT plus
code to make absolutely sure configure scripts that depended on a
shell variable internal to the original (2.13 era) definition of
AC_XENIX_DIR are not broken by autoupdate.  (That variable had the
temptingly public-sounding name “XENIX.”)  This compatibility code
uses AC_EGREP_CPP, which is itself discouraged for use in new
configure scripts.

(N.B. codesearch.debian.net does not find any uses whatsoever of
this macro, nor any code in an .ac or .m4 file that depends on the
XENIX variable.)

Change the compatibility code to use AC_CANONICAL_HOST instead,
and clarify which pieces of the code inserted by autoupdate are
probably still necessary.

* lib/autoconf/specific.m4 (AC_XENIX_DIR): Set XENIX variable
  based on value of host_os. Clarify what manual cleanup is
  recommended after autoupdate replaces this obsolete macro.