build-sys: fix update-potfiles script

Now are using "git ls-files" to avoid finding non-project
files or autoheaders which could break the dist tarball.

For non-git source dirs we assume that any existing POTFILES.in is
up-todate (build from tarball) or we provide at least an empty list
to not be a show-stopper for builds from poor "git archive".

Signed-off-by: Ruediger Meier <ruediger.meier@ga-group.nl>
tests: fix python tests for dist and out-of-tree builds

- add python helper scripts to the dist
- helper scripts are always in srcdir
- python libs are in builddir
- abort tests if helpers are missing

Signed-off-by: Ruediger Meier <ruediger.meier@ga-group.nl>
travis-ci: refactor and add .travis-functions.sh

Travis yaml syntax, where we can only use shell one-liners, is awful
and ugly. We add a real shell script and source it from .travis.yml.

This commit squashes a lot changings because we don't want to
pullote history with this meta CI stuff.

Highlights:
  - enable make distcheck
  - cleanup configure options (enable all progs, with python and docs)
  - shorter config.log dump
  - out-of-tree build supported
  - testing locally is possible now
  - easy to add temporary debugging stuff

Signed-off-by: Ruediger Meier <ruediger.meier@ga-group.nl>
travis-ci: workaround broken pylibmount relink

Signed-off-by: Ruediger Meier <ruediger.meier@ga-group.nl>