gtk3x: Link against gtk-3.99.0
Make gtk3x-client to build against gtk-3.99.0.
This is part of #43070 (Meta: Switch gtk4-client to use gtk-3.99.0), and will be pushed only when the whole of that meta ticket is ready for it.
This is not needed now, we have just one patch to go to 3.99.1 (no need for multiple tickets under a meta ticket)
Make gtk3x-client to build against gtk-3.99.0.
This is part of #43070 (Meta: Switch gtk4-client to use gtk-3.99.0), and will be pushed only when the whole of that meta ticket is ready for it.