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>