soup-message-io: Add sysprof profiling support for HTTP connections

This adds very basic support for dumping HTTP connection information to sysprof, if the process is being run under a sysprof session.

See sysprof#43 (closed) for plans of how this could be expanded in future. This is just a starting point.

The code in this commit dumps a message to the sysprof capture which includes the URI, total time for the connection (request + response), and the amount of data transferred in the request and response.

It adds an optional dependency on libsysprof-capture-4.a, and a subproject for building that if it’s not available on the system.

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