Until now, we weren't actually building TeXLive from source, but using the
TeXLive manager to download appropriate binaries. This doesn't allow us to
actually keep the source of the LaTeX packages (and build them offline for
example). For a fully (including the PDF) reproducible project, it is thus
necessary to have a controlled build of TeXLive packages from source.

With this commit this work has started. It is just important to note that
this work is not yet finished: we only install the base TeX installation,
we don't even have `latex' yet. Also the installation is set to be in a
non-standard directory (`$(idir)/textest') for testing purposes (so it
doesn't conflict with other installed software, enabling easy
checks).

Because of these in-completeness issues, the `./configure' script will
crash after the base TeXLive installation has been finished.