meson only supports the top level (no subdirs), so this makes the transition
easier.
For this to work with autotools, we need to *disable* the subdir-objects option,
that autotools wants us to enable for forward-compatibility.
This results in a bunch of warnings at autoreconf-time, but we don’t care, given
that we intend to switch away from autotools. Both build systems working next to
each other (as best as they can) is more important.
related to #4086
We now build a docker base container based on debian sid (where the very
latest packages are available). That base container is updated once a
month, or whenever travis-build.Dockerfile or debian/control change, but
re-used for subsequent travis runs. While the initial build might take
up to 15 minutes, subsequent builds typically run in a minute or two.
All the different steps that we run on travis are now factored into
separate scripts in the travis/ directory.
Switching to docker should also help with issue #2174.
