As rpm-ostree evolves, it keeps driving API additions to libostree.
This creates a relatively tight coupling.
However, if delivering via e.g. RPM, unless one manually remembers to
increment the `Requires:` in the spec file, it's possible for the two
to become desynchronized.
RPM handles versioned symbols and will ensure a dependency if the
application starts using a newer version.
To implement this, switch to `-fvisibility=hidden`, along with an
annotation in the header, and finally add a `.sym` file.
This matches what other projects like systemd and libvirt do.
Although rather than attempting to retroactively version symbols, glom
them all onto the current one.
A lot of effort here just to avoid touching SoupSession directly in
ostree_fetcher_new(). The reason will become apparent in subsequent
commits.
Note this introduces generated enum/flags GTypes using glib-mkenums.
I could have just made the property type as plain integer, but doing
properties right will henceforth be easier now that the automake-fu
is established.