Add Mesa to projects tested during continuous integration for C

[afd]

The CI for Dredd checks that it works for pinned revisions for a number of C projects.

As we are now working with Mesa, it would be good to add Mesa to this list, which would also serve to test tweaks that we are having to make for Mesa such as #351.

The C projects are tested via this shell script, so this is where support for Mesa should be added.

At a minimum we should check that some part of Mesa can be built successfully after being mutated.

Even better would be to run some Mesa unit tests after it is built, but we have to keep an eye on how long the CI takes to run, so this might be too time consuming to do during CI.