[Kde-bindings] FindQwt.cmake needs love

[Caleb Tennis]

This is a note somewhat to myself to take a look at this, but if someone else wants
to fix this in the meantime, please do so.

FindQwt.cmake does a good job, but it will happily find old Qwt versions that are
built against Qt3, and that causes a failure when kalyptus does its magic.

The easiest fix would be to find the headers and make sure that the Qwt version is
at least 5.0.0, but even then it's possible to build Qwt against Qt3 so some more
magical foo needs to be done.