Qwt 6.0.2 and 6.1.0 build cleanly for me using 4.8.0 and 4.8.4 and MingW on Windows. I don't have a Qt 4.8.1 to hand.

Try a clean build, using the Qt 4.8.1 qmake after commenting out the following line in qwtbuild.pri:
Qt Code: 
  1. CONFIG += silent
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BTW: Why use an old version of Qt?