This uses the new sysprof-3 ABI to implement the capture writer. It also
uses the statically linked libsysprof-capture-3.a that is provided with
Sysprof for the capture writing to ensure that we do not leak any symbols
nor depend on any additional libraries.

The GTK_TRACE_FD can be used to pass a FD for tracing into Gtk. Sysprof
uses this when the Gtk instrument is selected for recording.