You didn't install qwtpolar properly - instead you are trying to build against something left over in the build directory. Once having qwtpolar installed, you will see, that qwtpolar.prf ( CONFIG += qwtpolar ) is the one for building applications.
Have a look at http://qwt.sourceforge.net/qwtinstall.html. The concepts are the same for qwtpolar then.

Putting the sources of qwt and qwtpolar and your application into one project is not the officially supported way and then you have to be able to ship around those issues yourself.

Uwe