Fix bootstrap failure with the newest gnulib

With the newest gnulib, the Continuous Integration fails already in the 'bootstrap' phase [1]: bootstrap: running: /builds/gnu-m4/ci-distcheck/gnulib/gnulib-tool --no-changelog --no-libtool --symlink --update /builds/gnu-m4/ci-distcheck/gnulib/gnulib-tool: *** minimum supported autoconf version

Re: Crash on macOS in vasnprintf due to too old gnulib

Marvin Scholz wrote in: > I've encountered a crash in m4 that happens due to too old gnulib, as > its implementation of vasnprintf uses %n in a non-constant format string > which is not allowed on macOS since macOS 10.13 and leads