media-export: Fix GstDiscoverer error handling

The code was ported from old GUPnPDLNADiscoverer code under the assumption
that the error handling is the same (a bitfield), but it isn't.

https://bugzilla.gnome.org/show_bug.cgi?id=704317
build: Make missing gtk-doc non-fatal

Only fail if gtk-doc is missing when doc generation was requested explicitly
via --enable-valadoc

https://bugzilla.gnome.org/show_bug.cgi?id=692509
build: Rename --enable-valadoc to --enable-apidocs