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.

1 files changed, 38 insertions, 0 deletions

diff --git a/tests/torture.at b/tests/torture.at
index 631f34f4..f9bcf5e5 100644
--- a/tests/torture.at
+++ b/tests/torture.at
@@ -1926,3 +1926,41 @@ AT_CHECK([autoreconf -Werror -Wportability-recursive], 0, [], [])
 rm -rf configure config.h.in Makefile.in aclocal.m4 autom4te.cache
 
 AT_CLEANUP
+
+## ------------------------- ##
+## Missing auxiliary files.  ##
+## ------------------------- ##
+
+AT_SETUP([Missing auxiliary files])
+
+AT_DATA([configure.ac],
+[[AC_INIT([GNU foo], [1.0])
+AC_CONFIG_AUX_DIR([build-aux])
+AC_CANONICAL_HOST
+AC_OUTPUT
+]])
+
+AT_CHECK_AUTOCONF
+AT_CHECK_CONFIGURE([], [1], [ignore],
+[configure: error: cannot find required auxiliary files: config.guess config.sub
+])
+
+: > build-aux/config.guess
+
+AT_CHECK_CONFIGURE([], [1], [ignore],
+[configure: error: cannot find required auxiliary files: config.sub
+])
+
+AT_DATA([configure.ac],
+[[AC_INIT([GNU foo], [1.0])
+AC_CONFIG_AUX_DIR([build-aux])
+AC_PROG_INSTALL
+AC_OUTPUT
+]])
+
+AT_CHECK_AUTOCONF
+AT_CHECK_CONFIGURE([], [1], [ignore],
+[configure: error: cannot find required auxiliary files: install-sh
+])
+
+AT_CLEANUP