Improve autoconf-mode macro detection

Improve autoconf-mode macro detection

* doc/lispref/modes.texi (Search-based Fontification): Fix
indentation of (MATCHER . FACESPEC) example.
* doc/misc/cc-mode.texi (Performance Issues): Index
defun-prompt-regexp under variables, not functions.

* lisp/progmodes/autoconf.el (autoconf--symbol, autoconf--macro):
New rx definitions.
(autoconf-definition-regexp): Use an optional second capture group
to indicate a function rather than variable definition.  Detect
AC_DEFINE defining a function-like CPP macro.  Skip more shell
syntax such as variable ${} expansion and command `` substitution in
AC_DEFINE_UNQUOTED variable.  Match AH_VERBATIM, AM_CONDITIONAL, and
AM_MISSING_PROG as defining variables, and AC_DEFUN, AC_DEFUN_ONCE,
AU_ALIAS, and AU_DEFUN as defining functions.  Document first
capture group in docstring.
(autoconf-font-lock-keywords): Use autoconf--macro to match more
Autoconf macros, such as those defined in the Autoconf Archive and
Gnulib.  Reserve font-lock-function-name-face for function
definitions as determined by autoconf-definition-regexp, and use
font-lock-variable-name-face for the rest instead.  Use Font Lock
face symbols directly in place of their corresponding variable.
Fontify M4 changequote primitive only as a standalone symbol.
(autoconf-imenu-generic-expression): Add commentary mentioning new
submenu possibility.
(autoconf-current-defun-function): Update docstring accuracy.
Replace line-end-position with pos-eol since there are no fields.
(autoconf-mode): Define defun-prompt-regexp in terms of
autoconf--macro to support more toplevel macros, such as those
defined in Autoheader, M4sh, etc.  Set
open-paren-in-column-0-is-defun-start to nil to avoid false
positives when an Autoconf quote character is in column zero.

* test/lisp/progmodes/autoconf-resources/configure.ac: New file.
* test/lisp/progmodes/autoconf-tests.el
(autoconf-tests-current-defun-function-define)
(autoconf-tests-current-defun-function-subst): Replace character
motion with search.
(autoconf-tests-autoconf-mode-comment-syntax): Ditto.  Test both dnl
and # comments.  Use syntax-ppss-context.
(autoconf-tests-font-lock): New test.

(cherry picked from commit 2e3cf73e055be69d42a6f84436d400caf0997abb)