Error `Autoconf version 2.71 or higher is required` running `autoreconf -fi`

[ernstki]

The current master branch no longer builds on Ubuntu 20.04 (or derivatives, such as Zorin OS or elementaryOS) due to AC_PREREQ([2.71]) in configure.ac as of f1cd763.

Ubuntu 20.04 ("Focal Fossa") only has Autoconf 2.69.

$ lsb_release -a
lsb_release -a
No LSB modules are available.
Distributor ID: Zorin
Description:    Zorin OS 16.3
Release:        16
Codename:       focal

$ dpkg -l | grep autoconf
ii  autoconf      2.69-11.1      all      automatic configure script builder

When you attempt to run autoreconf -fi as instructed, the output will look something like this:

$ autoreconf -fi                                                                     
configure.ac:2: error: Autoconf version 2.71 or higher is required
configure.ac:2: the top level
autom4te: /usr/bin/m4 failed with exit status: 63
aclocal: error: echo failed with exit status: 63
autoreconf: aclocal failed with exit status: 63

Since Ubuntu 20.04 loses support in April 2024 (I think, going from memory here), probably the developers aren't going to want to do anything about this. I'll be closing the issue right after opening it, just so the workaround is recorded here for posterity.

Here's the workaround:

# check out the last version of `configure.ac` that _didn't_ require 2.71
git checkout -b ac-261 0bd1a07

# create a symlink from `README.md` to `README` to avoid the error
# `Makefile.am: error: required file './README' not found`
ln -s README.md README

# per the README
autoreconf -fi

From there, continue with the instructions in README.md