The problem are the wrong (back-)slashes in the path to the moc executable: "C:\Qt\4.3.1\bin/moc.exe".
With Qt 4.3.0 the support for qmake in the MSYS-shell has been added - but was buggy. (in the meantime set to fixed in the Qt Task Tracker). The workaround is to do a "qmake -r" in the qwt root directory.
I haven't tried it myself with Qt 4.3.1, but for problems like this, the reason is always because of qmake called from the MSYS-shell and a simple workaround is not to use the MSYS shell.
HTH,
Uwe
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