AC_PREREQ: update from 2.57 to 2.60 to reflect the de facto version used in configure.ac

[Gaetan Nadon]

This is effectively a noop. The real life minimum version of autoconf to
configure all modules is 2.60, which was released in 2006. For example,
AC_USE_SYSTEM_EXTENSIONS was introduced in 2.60. 

I have installed several versions of autoconf and automake and only
version >=2.60 was able to configure all modules. The tinderbox systems
are at 2.60 or greater. 

There is an assumption that we want to use the same level of autoconf
for all modules when we produce tarballs. 

Updating all modules will clarify what level we are at. One day we will
need to update autoconf to get some features we need, this will help.