Add a build description for gitlab's continuous integration build
and test runners. This gives us ongoing converage on commmits
and merge requests in the upstream repository.

Based on the gcc:9 image we currently use for most projects.
It already includes make and autotools, but we need to install
dependencies for the example programs and documentation.

  - debian buster has fig2dev in its own package.
  - latex-extras is needed for wrapfig, which has been superceeded
    by lwarp-wrapfig.

NB defining a separate job at the test stage clears all the
generated artefacts, which means we lose our configure script
and makefiles. Therefore isn't more efficient to just do the
tests as extra steps in the build stage.