info-look.el better autoconf support

[Kevin Ryde]

Some tweaks to better support autoconf 2.57.

        * info-look.el (autoconf-mode setups): Recognise AH_ and AU_ entries
        in "(autoconf)Autoconf Macro Index".  Add "(autoconf)M4 Macro Index"
        and "(autoconf)Autotest Macro Index".  Remove duplicate copy of
        "(automake)Macro and Variable Index".  Keep automake after all
        autoconf possibilities, so as to prefer those.

In an autoconf-mode buffer this make C-h C-i able to lookup more
things, like AH_VERBATIM, AU_DEFUN, m4_defn, AT_CHECK.

Unfortunately currently AT_CHECK and other Autotest index entries fail
on account of a period in the node name.  (Both info-look.el or
pressing Ret on an entry fail.)  Unless emacs is going to relax its
requirements about this I guess it should be fixed in autoconf.

info-look.el.autoconf-mode.diff
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