CI testing without GL dev packages
The current CI image comes with all EGL and GL -dev packages installed and headers installed into system include directories, which means that if a Weston build accidentally includes a EGL/GL/GBM header when it should not (e.g. EGL/GBM/gl-renderer disabled), or looks for a pkg-config file, it will find them and no error is raised.
I've pondered different ways to raise those errors, but the only non-hacky thing is to just have a CI docker image without those packages installed.
That means we would need to build two "layers": one image without Mesa dev packages, and another image based on the first one (an additional layer) that would have our current image contents.
Then we could change the new test case added in !282 (merged) to use the image without Mesa dev packages.
One reason why I wasn't fond of this approach before is that it probably doesn't scale - Weston has several optional dependencies and testing all combinations of each one present vs. missing is not feasible. Well, it's not feasible in the first place, with or without a docker image per configuration, so I guess that's not a reason to not test without Mesa dev packages.