Please add an option to fetch missing pieces online

[Jeffrey Walton]

Please add an option to fetch missing Autotools pieces online. Below,
there is no reason to torture me for these things. Autotools should be
able to find everything it needs on its own. If it can't find them
locally, then go get them from the network.

$ git clone https://github.com/libidn/libidn2.git
$ cd libidn2.git

$ git checkout libidn2-2.0.4
$ autoreconf -f -i

...
Copying file po/quot.sed
Copying file po/remove-potcdate.sin
aclocal: error: couldn't open directory 'unistring/m4': No such file
or directory
autoreconf: aclocal failed with exit status: 1