If a suitably new version of appstream-glib is available, use its
implementation of content rating systems (see
https://github.com/hughsie/appstream-glib/pull/364), rather than our
forked one.
This adds a dependency on libappstream-glib, but no particular version.
Eventually, our copy of `gs-content-rating.[ch]` can be dropped.
Signed-off-by: Philip Withnall <withnall@endlessm.com>
Fixes: #7
Some of the files in `malcontent-control` are GPL-2.0+.
Use the latest SPDX identifiers in the machine-readable data in
meson.build, but use the deprecated ‘+’ form in the README since it’s a
little more human friendly.
See https://spdx.org/licenses/.
Signed-off-by: Philip Withnall <withnall@endlessm.com>