FindQt4: Set the Qt4_FOUND variable if Qt4 is found

Provide the old QT4_FOUND for compatibility.
Merge topic 'set-Qt4_FOUND' into next

28d14f9 FindQt4: Set the Qt4_FOUND variable if Qt4 is found
FindQt4: Set the INTERFACE_QT_MAJOR_VERSION for Qt4::QtCore

With a similar commit in Qt 5, this will error at CMake time if
an attempt is made to use

 target_link_libraries(foo Qt4::QtCore Qt5::Core)
Merge topic 'set-QT_MAJOR_VERSION' into next

4aa10cd FindQt4: Set the INTERFACE_QT_MAJOR_VERSION for Qt4::QtCore