configure: GTK3 only. GTK2 is deprecated: if only GTK2 is found, warn and tell the user to install GTK3 (libgtk-3-dev / gtk3-devel). --with-gtk=2 just warns.
Co-Authored-By: Claude Sonnet 5.5 <noreply@...>
Authored by: mikeaubury 2026-10-16
Parent: [r13208]
Child: [r13210]