First of all please note that CMake >= 2.8.11 is required for building with
Qt5 support. In order to build LMMS with Qt5, add the following flag when
invoking cmake:

-DWANT_QT5=ON

If your Qt5 installation does not reside in standard installation paths,
additionally pass e.g.

-DCMAKE_PREFIX_PATH=/opt/qt53/