HAVE_STDBOOL_H, AC_HEADER_STDBOOL, and AM_STDBOOL_H

[Paul Eggert]

The gnulib stdbool module currently arranges to #define HAVE_STDBOOL_H if 
necessary,
but this symbol is never used.  This is confusing people who port Emacs
to systems that don't run 'configure', and I'd like to remove HAVE_STDBOOL_H
from Emacs's src/config.in file.

HAVE_STDBOOL_H is defined by Autoconf's AC_HEADER_STDBOOL (and thus by gnulib's,
for compatibility with Autoconf).  The only gnulib macro that uses 
AC_HEADER_STDBOOL
is gnulib's AM_STDBOOL_H; the latter name is curious, as it implies that the 
macro
belongs to Automake, but it's a gnulib macro.

It's a bit untidy.  How about the following idea to clean things up a bit?

In Gnulib:

* Rename gnulib's AC_HEADER_STDBOOL to gl_HEADER_STDBOOL.
* Remove the AC_DEFINE([HAVE_STDBOOL_H], ...) from gl_HEADER_STDBOOL.
* Rename gnulib's AM_STDBOOL_H to gl_STDBOOL_H.

In Autoconf:

* Mark AC_HEADER_STDBOOL as obsolescent, and suggest to
  Autoconf users that in the future they use gnulib if they want to
  be portable to pre-C99 hosts with respect to <stdbool.h>.