Warn when using a mkspec that might or might not work with the selected
tool chain.

Task-number: QTCREATORBUG-5854
Change-Id: Ifead5108ccd109f66707ad7aa371daead8cb57e7
Reviewed-on: http://codereview.qt-project.org/4522


Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Daniel Teske <daniel.teske@nokia.com>