ships a copy of libprojectm and doesn't provide configure option to use external copy
It would need us to apply these to projectm: http://code.google.com/p/clementine-player/source/browse/#svn/trunk/3rdparty/libprojectm/patches And the .patch files from here to qxt: http://code.google.com/p/clementine-player/source/browse/#svn/trunk/3rdparty/qxt
muh, is this still valid? And if so, why is it not fixed?
Because https://code.google.com/p/clementine-player/source/browse/3rdparty/libprojectm/patches/
Some of https://github.com/clementine-player/Clementine/tree/master/3rdparty/qxt patches were partly added to libqxt ebuild in tree, see bug #495984
At this point, libqxt is dead upstream and advises every project to pick the pieces they need and bundle those.