And add it to a new package set called 'qt5'. Later, we may add other
packages for GUI toolkits such as gtk3 and gtk4.

Users must set either QT5_PREFIX to point to their Qt installation, or
QMAKE to point to the qmake binary. When building android-universal,
users must set QT5_PREFIX since the qmake is different for each arch.

Tested on Linux, Android-Universal, iOS-Universal, macOS, Windows