docs: remove obsolete references to Autoconf 2.13

docs: remove obsolete references to Autoconf 2.13

Autoconf 2.13 is definitely obsolete today, so giving hints or caveats
about it in the Automake documentation is not only obsolescent, but
also counter-productive, as it suggests that autoconf 2.13 is still
relevant.

* doc/automake.texi: Adjust.  Related rewordings.  Since we are at it,
prefer using a more modern autoconf version number (2.68 instead of 2.57)
in an example showing how to correctly use 'AC_PREREQ' in third-party
.m4 files.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>