Improve handling of missing aux scripts.

Improve handling of missing aux scripts.

Another regression identified by the Debian archive rebuild was that
more macros require the presence of config.sub and config.guess now.
‘autoreconf --install’ doesn’t install these itself, it relies on
‘automake --add-missing’ to do that; so, packages that don’t use
Automake will fail at the configure stage after configure is
regenerated.  To make matters worse, AC_CONFIG_AUX_DIRS assumes that
everyone who needs config.sub and config.guess also needs install-sh,
so in about half of the affected packages, the failure manifested as a
complaint about install-sh being missing -- technically true but
adding install-sh wouldn’t have resolved the problem by itself.

This patch overhauls the AC_CONFIG_AUX_DIR(S) mechanism so that a
configure script knows the complete set of aux scripts that were
AC_REQUIRE_AUX_FILE’d for it, checks for the existence of all of
them, and not any others.  Thus, this configure script

    AC_INIT([test], [1.0])
    AC_FUNC_MALLOC
    AC_CONFIG_HEADERS([config.h])
    AC_OUTPUT

will work fine in a directory that contains config.sub and
config.guess but not install-sh.  Also, if it’s in a directory
that *doesn’t* contain config.sub and config.guess, it will print an
accurate error message

    configure: error: cannot find required auxiliary files: config.guess config.sub

instead of the misleading

    configure: error: cannot find install-sh, install.sh, or shtool in "." "./.." "./../.."

A side-effect: it doesn’t make sense for AC_CONFIG_SUBDIRS to demand
the presence of Cygnus configure in the aux dir, on the off-chance
that one of the subdirectories *might* be using it -- I have no idea
where someone would even get a copy of that nowadays -- so I dropped
that feature.  I rather suspect nobody has needed it in over a decade.

I also documented the expanded need for config.sub and config.guess in
NEWS as well as the manual.

* NEWS: Document expanded need for config.sub and config.guess.
  Document removed support for Cygnus configure in subdirectories.

* doc/autoconf.texi: Clarify exactly when install-sh, config.sub,
  and/or config.guess are required.  Document canonical online sources
  for these scripts.  Revise documentation of AC_CONFIG_AUX_DIR and
  AC_REQUIRE_AUX_FILE.  Minor improvements to documentation of
  AC_CONFIG_SRCDIR.  Remove mentions of Cygnus configure in
  subdirectories.

* lib/autoconf/general.m4
  (_AC_INIT_PARSE_ARGS): Remove mention of Cygnus configure;
  clarify function of configure.gnu.
  (AC_CONFIG_AUX_DIR): Support multiple invocations.
  (AC_CONFIG_AUX_DIRS): Now an undocumented compatibility interface
  rather than an internal subroutine; just runs AC_CONFIG_AUX_DIR on
  each of its arguments.
  (AC_CONFIG_AUX_DIR_DEFAULT): Now a backward compatibility stub that
  requires _AC_INIT_AUX_DIR without adding anything to _AC_AUX_FILES.

  (AC_REQUIRE_AUX_FILE): Now adds the named aux file to _AC_AUX_FILES
  and requires _AC_INIT_AUX_DIR, as well as being a trace hook.

  (_AC_INIT_AUX_DIR): New home of the loop searching for necessary aux
  files (formerly in AC_CONFIG_AUX_DIRS).  Looks for all the necessary
  aux files, not just for install-sh.

  (ac_config_guess, ac_config_sub, ac_configure): Issue deprecation
  warnings if these undocumented shell variables are actually used.

  (AC_CANONICAL_BUILD, AC_CANONICAL_HOST, AC_CANONICAL_TARGET):
  No need to require AC_CONFIG_AUX_DIR_DEFAULT.
  Can rely on $ac_aux_dir ending with a slash.

  * lib/autoconf/programs.m4 (AC_PROG_INSTALL, AC_PROG_MKDIR_P):
  No need to require AC_CONFIG_AUX_DIR_DEFAULT.

  * lib/autoconf/status.m4 (_AC_CONFIG_SUBDIRS):
  No need to require AC_CONFIG_AUX_DIR_DEFAULT.
  Remove check for Cygnus configure; clarify function of configure.gnu.

  * lib/autotest/general.m4: Remove mention of Cygnus configure.

  * tests/torture.at (Missing auxiliary files): New test.