FindQt

Searches for all installed versions of Qt.

This should only be used if your project can work with multiple versions of Qt. If not, you should just directly use FindQt4 or FindQt3. If multiple versions of Qt are found on the machine, then The user must set the option DESIRED_QT_VERSION to the version they want to use. If only one version of qt is found on the machine, then the DESIRED_QT_VERSION is set to that version and the matching FindQt3 or FindQt4 module is included. Once the user sets DESIRED_QT_VERSION, then the FindQt3 or FindQt4 module is included.

This module can only detect and switch between Qt versions 3 and 4. It cannot handle Qt5 or any later versions.

QT_REQUIRED if this is set to TRUE then if CMake can
            not find Qt4 or Qt3 an error is raised
            and a message is sent to the user.

DESIRED_QT_VERSION OPTION is created
QT4_INSTALLED is set to TRUE if qt4 is found.
QT3_INSTALLED is set to TRUE if qt3 is found.