Merge relevant changes from Skeletonxf's GitLab.

[DaloLorn]

Some of the changes have been rendered redundant, either by changes in other repos (the ringworld fix, for instance), or by modification of the repo structure. However, there did still seem to be a few items that could be useful.

[Skeletonxf]

I made a start on a shell script that installed all the apt dependencies required to build on Ubuntu. That would be a useful starting point for the CI setup, but I don't have any experience in setting up automated builds myself.

[DaloLorn]

Hopefully @kb-1000 will be able to help with that... in which case the script could be of use to him.

In the meantime, it can still be useful to help Linux devs set up their dev environments.