Use MacPorts's ggdb on macOS

[sampotter]

If you install gdb using macOS, it gets renamed to ggdb. (E.g., see this SO post.)

It would be helpful to have cgreen-debug check for this automatically, which can be done easily by switching the last few lines of the script to:

if command -v cgdb ; then
    debugger=cgdb
elif command -v ggdb ; then
    debugger=ggdb # MacPorts renames gdb "ggdb" to avoid name collisions
else
    debugger=gdb
fi

(This issue is a reminder to myself to fix this, which I plan on doing once #246 is merged.)