The qtquick-config.h file was being generated empty by the cmake
build, causing 'division by zero in #if' errors in downstream
consumers (SDDM, KWin) because QT_CONFIG(quick_shadereffect) and
QT_CONFIG(quick_draganddrop) were undefined.
Two fixes:
1. Explicitly enable QT_FEATURE_quick_shadereffect and
QT_FEATURE_quick_draganddrop in the cmake configuration.
2. Add a safety net that regenerates qtquick-config.h with the
required feature definitions if cmake produces an empty file.
