OK ... I went to the Projects view, clicked on "Build & Run", then "Manage Kits", cloned the auto-detected "Desktop Qt 5.3" kit, then made sure that the GDB in /usr/local/bin was selected, then clicked on "Make Default" at the right.
BINGO!! :))
Now everything is working as it should, and I see 7.8.1 for the GDB version in the debugging output at the top.