RFC: Bump minimum Perl to 5.18.0 for next major release of both Autoconf and Automake

[Zack Weinberg]

I propose that, as of the next major feature release of both Autoconf
and Automake, we raise the minimum version requirement for Perl to
5.18.0.  (Currently it is 5.6.0.)  On the Autoconf side, the version
number and timeframe for the first release with this change are still
TBD but it would *not* come from the post-2.70 release branch.  I
don't know what the Automake maintainers' current release schedule
looks like but I imagine this would be targeted for 1.17.0, whenever
that happens (ideally at about the same time as the Autoconf release
making the same change).

Increasing the minimum version requirement would have these benefits:

 - Access to useful library modules, notably Digest::SHA, List::Util,
Storable, and Time::HiRes.
 - Comprehensive interpreter support for Unicode (specifically, 'use
feature "unicode_strings"').  Neither autoconf nor automake makes much
use of non-ASCII characters themselves, but having Unicode supported
in the interpreter would make it easier for us to be sure that we
don't mess up non-ASCII text in our inputs.
 - Any number of bug fixes; I have personally tripped over problems
with File::Spec and File::Temp in 5.6.0.
 - Any number of performance enhancements.

Perl 5.18.0 was released in May 2013.  (Perl 5.6.0 was released in
March 2000.)  I selected it as the minimum because the Perl
maintainers recommend all users of older interpreters upgrade to at
least this version, due to security fixes (see
https://perldoc.perl.org/perlsec#Algorithmic-Complexity-Attacks --
inputs to Autoconf and Automake are trusted, but this rationale means
it's hard to find a CI platform that offers anything older than 5.18,
and we ought to be testing our minimum version requirement.)  I'm open
to arguments for setting it either lower or higher.

zw