Make sure we can configure CMake if Qt 4 is not available but Qt 5 is.

 find_package(Qt4 QUIET)

should be non-FATAL in that case. Also, make sure we always find the
FindQt4.cmake shipped with CMake itself.
Merge topic 'fix-build-without-qt4-with-qt5' into next

1a1a7ae Make sure we can configure CMake if Qt 4 is not available but Qt 5 is.