A recent libxml2 changed its handler function to take a const pointer. It's

  safe for us to have it, and them not, but the opposite way causes an implicit

  cast warning.
In relatively new GLib (many years now), g_object_ref casts its output to 

  match its input. This means we should not be casting to G_OBJECT, as that

  is not the type it expects, and would translate to the output being GObject

  instead of the original type.

This fixes the build in Fedora 40, which changes several incompatible pointer

conversion warnings into errors.
This patch is partially from the Fedora maintainer @yarda, with some corrections by me (to the g_object_ref portions.)

Compiled in a Rawhide environment with this patch applied.