Add gitlab ci configuration.
Add gitlab ci configuration.

Describe a basic autoconf build and test for gitlab's integrated
continuous integration runner. Uses the gcc docker image.

Copied from the theora version.

- `zip` is needed for `make dist`
- `doxygen` is needed to generate api documentation.
  latex is also needed, but a heavy dependency to install every run,
  so skipped for now.

ShiftMediaProject/vorbis,ShiftMediaProject/vorbis,ShiftMediaProject/vorbis,ShiftMediaProject/vorbis,ShiftMediaProject/vorbis,ShiftMediaProject/vorbis