Yes, indicating that "CONFIG += qwt" didn't work because your qmake is not configured properly ( you can prove this easily by replacing "CONFIG += qwt" by "include ( C:\Qwt-6.1.0\features\qwt.prf )".
You wrote, that you have set QMAKEFEATURES - what exactly is the output of "qmake -query QMAKEFEATURES" and are you sure, that this is the same qmake version as it is used by the creator ?
Uwe
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