on debian testing there is nothing special to be done for installing qt5.
but you need the qt-5 dev package. at least, you probably need qtbase5-dev.
on debian testing my cmake command looks like this:
cmake -DCMAKE_INSTALL_PREFIX:PATH=/home/ale/docs/bin/scribus -DWANT_DEBUG=1 -DWANT_GUI_LANG="en_GB;de;fr;it;en" ..
you might want to add -DWANT_NOOSG=1 if osg is installed (as far as i recall, cmake or make crashe in a bad way if it finds osg... and you probably don't need it in scribus)
you don't need any QT5 path directive in the cmake.
and i'm pretty sure that there is nothing you should change in the code!
it's about getting the right packages and not putting "ill" commands in cmake...
danielsan: at this point, i think the best is an interactive session, where we try in parallel to set it up...
i can be available over lunch on http://wiki.scribus.net/canvas/Special:WebChat
if you tell me that you'll be there.
----> this seems to be a bit broken, too:
if you're already on irc: get into #scribus on freenode.
otherwise on the web interface above, write the two commands:
<----- sorry, i'm not in charge of that :-(