Add .gitlab-ci.yml

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.

As the version of wayland-protocols available is not sufficiently new,
we clone and build our own local version first. libwayland is new
enough, however we could potentially reuse the artifacts generated by
the Wayland CI job.

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/weston/pipelines/

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