build: add GNOME GitLab CI configuration

The pipeline begins with "build" stage doing a distcheck on Fedora 28
(which is still known to ship libnm-glib) and outputting a tarball
artifact.

The output is then used in the "test" stage.

In future, builds on some older platforms, CentOS and Ubuntu and clang
builds would be nice. Not implemented at this point, but it should be
straightforward enough.

Maybe a build with a Git snapshot of NetworkManager and/or
network-manager-applet would be useful at some point, but that's not
implemented either.
2 jobs for lr/ci in 12 minutes and 16 seconds (queued for 2 seconds)
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