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. This fixes #14142.
Make sure we always find the FindQt4.cmake shipped with CMake itself.
Merge topic 'fix-build-without-qt4-with-qt5' into next

72bd206 Make sure we always find the FindQt4.cmake shipped with CMake itself.
64e262b Make sure we can configure CMake if Qt 4 is not available but Qt 5 is.