Add GitLab CI configuration.

Add a .gitlab-ci.yml file to run libglvnd's unit tests.

The "build-distcheck" target will simply run "make distcheck". That should also
cover running the unit tests for the x86-64 TLS build.

The "build-i386", "build-i386-tsd", and "build-x86-64-tsd" targets will build
and test the x86 TLS and TSD builds and the x86-64 TSD build.

The pure C stubs aren't covered yet, but they'll be easy to add once the unit
tests are fixed for them.

ARM and PPC builds also aren't covered yet.