IDN2 and syntax error... PKG_CHECK_EXISTS($gtk_doc_requires, have_gtk_doc=yes, have_gtk_doc=no)'

[Jeffrey Walton]

I'm experiencing an configure error for IDN2 on three different
machines. The issue is present in
https://ftp.gnu.org/gnu/libidn/libidn2-2.0.4.tar.gz :

checking for alloca as a compiler built-in... yes
checking whether the compiler generally respects inline... yes
checking for ssize_t... (cached) yes
./configure: line 25423: PKG_PROG_PKG_CONFIG: command not found
checking for gtk-doc... ./configure: line 25430: syntax error near
unexpected token `$gtk_doc_requires,have_gtk_doc=yes,have_gtk_doc=no'
./configure: line 25430: `
PKG_CHECK_EXISTS($gtk_doc_requires,have_gtk_doc=yes,have_gtk_doc=no)'
gmake: *** [cfg.mk:56: buildit] Error 2
Failed to build IDN2

Autotools and Libtool have been updated:

    $ autoconf --version
    autoconf (GNU Autoconf) 2.69

    $ aclocal --version
    aclocal (GNU automake) 1.15.1

    $ automake --version
    automake (GNU automake) 1.15.1

    $ command -v autoconf
    /usr/local/bin/autoconf

    ...

I don't enable docs, so I'm happy if they are just skipped. They are a
waste of time and space because these are dev machines.

Any ideas how to avoid the configuration problems?

Thanks in advance.