Merge remote-tracking branch 'upstream/3.4' into merge-3.4

diff --cc cmake/OpenCVCompilerOptions.cmake
Simple merge

diff --cc modules/core/include/opencv2/core/mat.inl.hpp
Simple merge

diff --cc modules/highgui/src/window_QT.cpp
Simple merge

diff --cc modules/videoio/src/cap_v4l.cpp
Simple merge