FFmpeg: 3.3.4 -> 3.4.2
LibVPX: 1.6.1 -> 1.7.0
OpenH264: 1.6.0 -> 1.7.0

- ffmpeg 3.3 -> 3.3.2
- added VERSIONINFO

ffmpeg 3.2.1 -> 3.3
libvpx 1.6.0 -> 1.6.1

- more aggressive cache for files under CMAKE_BINARY_CACHE
  * don't re-read files MD5
  * don't repack archives
- add support for ENV{OPENCV_DOWNLOAD_PATH}
- added ID parameter as project/group identifier
- non-flat .cache directory (based on ID)
- download message prefix based on ID
- more detailed logging via ocv_download_log() macro
- force .gitignore file for .cache folder (with '*' pattern)

Versions:
- ffmpeg 3.2.1 (was 3.1.3)
- openh264 1.6.0 (was 1.5.0)
- libvpx 1.6.0 (was 1.5.0)

Scripts are updated for Linux-based (Ubuntu 14.04) mingw cross-compilation (full stack of scripts provided)
Part of these scripts may work under Windows installation of MinGW, but it is not supported.

FFMPEG update: 2.7.1
Added OpenH264 Cisco binaries support for H264 encoding: v1.4.0

(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

2. Algorithm::load/save added (moved from StatModel)
3. copyrights updated; added copyright/licensing info for ffmpeg
4. some warnings from Xcode 6.x are fixed

FFmpeg 2.0.2 32-bit builded with mingw (4.6.1) with following additional options: --enable-w32threads --disable-iconv --cpu=i686 (workaroud for http://trac.ffmpeg.org/ticket/2363)

While the FFmpeg video IO backend gets compiled even without it,
the tests (for both FFmpeg and video IO) don't. This should fix
that.

folowing additional options: --enable-w32threads, --arch=i686 (for 32-bit
library, workaround for http://trac.ffmpeg.org/ticket/2363)