Changing search order for qwt

On gentoo the qwt5 lib is named qwt and the qwt6 lib is named qwt6.
So the  actual FindQwt.cmake results in 

//Path to a file.
QWT_INCLUDE_DIRS:PATH=/usr/include/qwt6

//Path to a library.
QWT_LIBRARIES:FILEPATH=/usr/lib64/libqwt.so

That results in 

build/gr-qtgui/lib/libgnuradio-qtgui-3.7.3git.so.0.0.0: undefined symbol: _ZNK7QwtPlot9drawItemsEP8QPainterRK6QRectFPK11QwtScaleMap

when running make test.

Changing the search order ( first qwt6 then qwt ) fixes this problem.