Enable traces; wipe out the old configure.in parser.

Enable traces; wipe out the old configure.in parser.

* automake.in (scan_autoconf_traces): Use '$ENF{AUTOCONF}' of
'autoconf'.  Honnor the $filename argument.
(scan_autoconf_files): Don't call scan_one_autoconf_file.
Always call scan_autoconf_traces.
(scan_one_autoconf_file): Remove, with it associated regexes.
(obsolete, obsolete_rx): Remove.
(register_language, handle_languages): Remove the 'define_flag'
support.  Remove it from the struct and language definitions too.
(unquote_m4_arg): Remove.
* lib/am/header-vars.am, lib/am/compile.am, lib/am/lisp.am: Remove
the `FOO = @FOO@' definitions for all AC_SUBST'ed variables.  They
are now generated automatically.
* aclocal.in (obsolete_macros): Don't mention obsolete_macros in
automake.in.
* automake.texi (Invoking Automake): Mention the AUTOCONF envvar.