Update to 4.7.2.

This is a minor bug fix release:

• Fix CMake 3 compatibility by synchronizing the FindQt4.cmake module from kdelibs git.
• Prevent possible internal infinite loop call chains when trying to handle backend construction in simulation mode (i.e. without actual backend being installed on the system).
• When not building with PHONON_INSTALL_QT_EXTENSIONS_INTO_SYSTEM_QT, make sure that the relative Qt paths are set according to whether the build is meant for Qt4 or 5. Previously this would always install to a path for qt5.