Add .gitlab-ci.yml

Add a GitLab CI configuration which tests building, 'make check', and
'make distcheck' of the tree inside a Debian Stretch container. The
choice of distribution base was arbitrary and may easily be changed.

When commits are pushed to upstream, the commits will run this CI
pipeline to run these tests, and capture the result as an artifact
bundle, including the compiled binaries and full test suite logs.

Results can be seen at:
    https://gitlab.freedesktop.org/wayland/wayland/pipelines/



Signed-off-by: Daniel Stone <daniels@collabora.com>