Allow using g_log_structured instead of g_log
Look for a macro G_LOG_USE_STRUCTURED, and if it is defined, use
g_log_structured instead of g_log when defining g_warning and friends.
This avoids the extra complication of going through g_logv _and_
g_log_structured to get a message logged; it also lets us pass
the code-related fields.
We don't do this unconditionally (yet), since some users might
rely on the more fine-grained fatality support in g_logv. It has
also been proven problematic in the past to inject a dependency
on bleeding-edge API via a widely-used macro.
https://bugzilla.gnome.org/show_bug.cgi?id=744456