Add basic configuration for Gitlab CI
This Merge Request adds a basic CI configuration. For now it tries to build Gambas on the following distributions:
- ArchLinux (rolling)
- Debian (oldstable, stable, testing, unstable)
- Ubuntu (latest, xenial, trusty)
Note builds on some older Debian are already failing because of Poppler, but this can be merged as-is, to quickly check for potential regressions before making a new release.