disable Ctrl-C stub in debug build

Activating the controlcstub in a debug build will send Ctrl-C to
Qt Creator too, because he stub and Qt Creator share the same
Windows.
We're disabling the stub in debug builds. Cancelling a build
still works but takes a bit longer.

Task-number: QTCREATORBUG-11995
Change-Id: I766c2eac1f07205c411a7e4164e7d73b6c0441aa
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>