GitLab

Make function to download and unpack archives.

This allows caching the archives in the `archives` directory, either by
committing to the git repo, or when doing repeated builds of the same
library.

Also, enable encoding on basis of
https://gitlab.dkrz.de/k202009/libaec/blob/master/doc/license.txt

Make configuration more general.

Move responsibility for source futzing back to the travis.yml file,
because we are now doing only one build per job.