Don't treat warnings as errors.

It's too much hassle if e.g. the warnings come from header files.

Task-number: QTCREATORBUG-11431
Change-Id: I12af8b4a2bc525f2d7c415659e9a51f8e92790e6
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>