Go back to requiring only Perl 5.6+ for users

Go back to requiring only Perl 5.6+ for users

Commit 61901a1a14fd50c03cfb1529d091554376fef286 dated 2022-07-10
bumped the Perl requirement to 5.10 or later, because
commit 3a9802d60156809c139e9b4620bf04917e143ee2 dated 2021-08-31
added code using Time::HiRes’s ‘stat’ function, a feature
added in Perl 5.8.9+ or Perl 5.10+, and it was hard
to find Perl 5.8.9 hosts to test with.  Also, requiring Perl 5.10
meant that we could then use operators like Digest::SHA, the // and
//= operators, the regexp \K escape, and ‘state’ variables.

However, that Time::HiRes code, which was taken from Automake, has
recently been made optional by Automake, and it now works again with
Perl 5.6.  And Autoconf is not yet using any other post-5.6 feature,
except when developers run help-extract.pl (something Autoconf users
do not use).  So relax the Autoconf user requirement back to 5.6 as it
was in Autoconf 2.71; although Autoconf developers will need 5.10 or
better, Autoconf users can get by with 5.6.

I ran into this problem when testing the Autoconf release candidate on
Solaris 10, which has Perl 5.8.4.  Oracle says Solaris 10’s
end-of-life is January 2024, so it’s still (barely) a viable porting
target.  Of course with Solaris 10 one must install a recent-enough
GNU m4, but adding a requirement to also install a recent-enough Perl
is a new barrier, and if it’s not needed then it might be better to
wait until it is needed (or until 2024 arrives).

* NEWS: Update news item about Perl 5.6 vs 5.10.
* README-hacking: Bump Perl recommendation to 5.10.
* build-aux/fetch.pl: Do not munge imported code to require 5.10.