All consumers of the submodule switched to meson, except the CI build.
It neither seems useful to maintain a second build system just for that
purpose, nor to test a configuation that isn't used by anybody.
So set up a small fake project that includes gvc as a subproject, and
build that during CI.
https://gitlab.gnome.org/GNOME/libgnome-volume-control/merge_requests/9
