CI fixes

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.