* doc/autoconf.texi: Fix some systematic formatting problems.

* doc/autoconf.texi: Fix some systematic formatting problems.
".)"  needs a following @: if not at the end of a sentence, and
similarly for "!)".  "etc." should be preceded by a comma.
"n-th" -> "@var{n}th".  pdksh is still buggy, so update its date.
(Input): Mention that AC_CONFIG_AUX_DIR's
argument is often called 'build-aux'.