Add initial sysprof support
This adds initial support for sending profiling data to sysprof. Mostly, it sends data from
GSource, but other sources could be added in future.
At the moment it adds an optional dependency on
libsysprof-capture-3. This is non-optimal, because
libsysprof-capture-3 depends on
libgio, so there’s a cyclic dependency.
My suggestion is that
libsysprof-capture-3 is rewritten to drop its
libgio dependencies, but that’s very much open for discussion. @chergert may have better ideas than I have had.
See the various commit messages for more details.