Turn AC_FUNC_SETVBUF_REVERSED into a noop. It's been obsolete for
years and is too hard to maintain now. The last straw was
reported by Jerker Baeck in
<http://lists.gnu.org/archive/html/autoconf/2006-11/msg00102.html>.
* NEWS: AC_FUNC_SETVBUF_REVERSED is now obsolete.
* doc/autoconf.texi (Particular Functions): Move
AC_FUNC_SETVBUF_REVERSED from here...
(Obsolete Macros): ... to here. Say that it does nothing now.
* lib/autoconf/functions.m4 (AC_FUNC_SETVBUF_REVERSED):
Turn into (almost) a no-op.