teach setup-qt how to detect broken Qt packages
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 <3aa99dcdd3f0cac61fb81c2a11771c0cc2497607@digia.com>
Reviewed-by: Joerg Bornemann <47777b9406dbddd02a3fdca8341789a013805a11@digia.com>

gatzka/qt-labs-qbs,villytiger/qbs,qt-labs/qbs,Distrotech/qbs,qt-labs/qbs,Distrotech/qbs,villytiger/qbs,gatzka/qt-labs-qbs,qtproject/qt-labs-qbs,Distrotech/qbs,Distrotech/qbs,qtproject/qt-labs-qbs,villytiger/qbs,Distrotech/qbs,qtproject/qt-labs-qbs,gatzka/qt-labs-qbs,villytiger/qbs,qtproject/qt-labs-qbs,qt-labs/qbs,qtproject/qt-labs-qbs,qtproject/qt-labs-qbs,villytiger/qbs,qt-labs/qbs,qt-labs/qbs,gatzka/qt-labs-qbs,qtproject/qt-labs-qbs,Distrotech/qbs,qt-labs/qbs,qt-labs/qbs,villytiger/qbs,gatzka/qt-labs-qbs,gatzka/qt-labs-qbs,Distrotech/qbs,gatzka/qt-labs-qbs