Keep settings not found in the SDK-installed kit as seen in the
user-specific settings, but use the SDK settings for everything
else.

Downside: The compilers might change if you have several compilers
supporting the same ABI.

Change-Id: I71a0935cc180149b333ec38a03bd2556892c49cc
Tasknumber: QTCREATORBUG-8390
Reviewed-by: Daniel Teske <daniel.teske@digia.com>