$(add_qt_dep qtpositioning) is missing in dependencies.
at a bare minimum, missing build.log.
I downgraded from qt-5.12.1 now (as I was not able to get working KDE with it on nouveau) where Positioning is present and Falkon depends on it. On qt-5.11.3 it is not part of qt and falkon ebuild needs no modification.
Still missing build.log, which would have been quite interesting as Qt 5.12 will inevitably be moved to tree at some point.