If you use Qt 5, use -DLYX_USE_QT=QT5, otherwise Qt 4 will be searched.
When you want to run LyX without installing from an out-of-source
If you use Qt 5, use -DLYX_USE_QT=QT5, otherwise Qt 4 will be searched.
When you want to run LyX without installing from an out-of-source