opening a folder is system specific - made system specific code
added license

Backport of mariusmuja/flann@f8ca6df3.

https://bugs.launchpad.net/ubuntu/+source/gcc-4.9/+bug/1474304

A define was forgotten for the codecs and there was a typo for
another.

(cherry picked from commit f052b0bc)

Conflicts:
	modules/videoio/src/ffmpeg_codecs.hpp

(cherry picked from commit 30912b49)

Conflicts:
	modules/videoio/src/cap_ffmpeg_impl.hpp
	modules/videoio/src/ffmpeg_codecs.hpp

(cherry picked from commit 2b2e3ee9)

Conflicts:
	cmake/OpenCVFindLibsVideo.cmake
	modules/videoio/src/cap.cpp
	modules/videoio/src/cap_ffmpeg_impl.hpp
	modules/videoio/src/ffmpeg_codecs.hpp
	modules/videoio/test/test_ffmpeg.cpp