teach setup-qt how to detect broken Qt packages

Qt4 packages for MinGW have an invalid QMAKESPEC_ORIGINAL set.
See QTBUG-28792. We must work around this issue.

Change-Id: Ibf41e4a8eaed8aa5a7855693aaf653fc5d323cd1
Task-number: QBS-629
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>