All works well, except the new version cannot find gdb - although 2304 can - even if I set the debugger path under Settings -> Debugger Settings .. -> Debugger Path: C:\MinGW-3.4.5\bin\gdb.exe
All other options in this dialog are unchecked.
After a bit of looking and testing, I found that CL depends on finding the GNU version of which in the Runtime directory.
If I move a copy to that directory, it then can find gdb and I can debug my program.
If CL can't find which in Runtime, it looks in the places listed on PATH, and in my case, it found a version of "which", but did not seem happy with the version it found and I guess eventually gave up
I'm not sure this is really a bug, but for future reference and to help anyone running into the same problem, I thought I'd leave a 'trail'.
On second thought, though, it is a bug, since even if I delete the path in the debugger settings dialog, CL still uses the same process to try and find gdb and if it doesn't find the which it likes, it fails to find the debugger.