[NITPICK] build: AC_PREREQ minor issues

[Stefano Lattarini]

I see that Texinfo's configure.ac requires autoconf 2.59 with
an AC_PREREQ call, after AC_INIT and AM_INIT_AUTOMAKE has been
invoked.  But AM_INIT_AUTOMAKE requires Automake 1.12 or later,
and that will cause autoconf 2.62 or later to be AC_PREREQ'd
anyway, so that the explicit AC_PREREQ by Texinfo is redundant.
IMHO, it should be either removed, or made consistent with
what AM_INIT_AUTOMAKE([1.12]) requires.

Regards,
  Stefano