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.