Add some internal wrappers around sysprof tracing, so that it can be
used throughout GLib without exposing all the details of sysprof
internally.

This adds an optional dependency on `libsysprof-capture-4`. sysprof
support is disabled without it.

This depends on the GLib dependency of `libsysprof-capture` being
dropped in sysprof!30

,
which has bumped the soname of `libsysprof-capture` and added subproject
support.

The next few commits will add marks that trace out each `GMainContext`
iteration and each `GSource` `check`/`prepare`/`dispatch` call.

Signed-off-by: Philip Withnall <withnall@endlessm.com>