libtar: 1.2.20

Update libtar to 1.2.20, add build dependencies on autotools.
Point to new repository where development has been taking place.

The [libtar homepage](http://www.feep.net/libtar/) is down right now, but the [git repository](http://repo.or.cz/w/libtar.git) that hosts the latest development still lists it as the project homepage.