Outdated libtool creates trouble on macOS

[Marc Glisse]

On Sun, 27 Aug 2023, Torbjörn Granlund wrote:

> We have been using the default shell.gmplib.org autotools tools for a
> week now, for generating the public snapshots and the nightbuild
> snapshots.  Things look good.

Do you think we could increase AC_PREREQ to 2.70 (or even 2.71), which 
would allow a couple more cleanups? I don't think we want to support users 
rerunning autoconf with an older version than the one we use to generate 
the tarballs.

-- 
Marc Glisse