add default OSX homebrew path to FindQwt ...

Add them last in the list; should never conflict with other OSs nor MacPorts or Fink.  I've tested in MacPorts and there's no issue (MacPorts does not install Qwt as a framework; hasn't for a long time if ever). I have no way to test with Fink, but it by default installs into /sw so this should not be an issue.

Should address gnuradio bug #642 < http://gnuradio.org/redmine/issues/642 >.