gitlab-ci: Add build configs.
Define builds for gitlab's continuous integration runners so we some feedback on versions of the code hosted there, particularly https://gitlab.xiph.org/xiph/flac. Build with GNU Autotools, with CMake+ninja, and verify the Makefile.lite build. This uses the same Debian-based docker.io/library/gcc:9 container image we use for most of the codec projects. That container doesn't specify an unprivileged user so that we can install packages. However, some of flac's file-creation tests must be run as an unprivileged user who owns the source tree. We therefore create a user account and use `su` to run the tests. In the case of the autotools job, `make distcheck` unpacks the packaged source, so this takes care of ownership. In the case of the other jobs, we must `chown` the entire checkout and complete the build as the unprivileged user. Another way to address this would be to define a custom container image with the prerequisites installed before switching to an unprivileged user for the entire build. The current approach was simpler to get working.