FindQt4: Fix QUIET failure with Qt 5 but not Qt 4

The call

 find_package(Qt4 QUIET)

should be non-FATAL in that case. This fixes #14142.
Merge topic 'fix-build-without-qt4-with-qt5' into next

444e752 FindQt4: Fix QUIET failure with Qt 5 but not Qt 4
Error on relative path in INCLUDE_DIRECTORIES target property.

Add policy CMP0021 to preserve existing behavior in projects expecting
it from earlier CMake versions.
Merge topic 'error-on-exported-missing-include-dir' into next

eabefa8 Error on relative path in INCLUDE_DIRECTORIES target property.