- Recommended compiler checks:
  - GCC: CV_GCC
  - Clang: CV_CLANG
- fixed problem with CMAKE_CXX_COMPILER_ID=Clang/AppleClang mess on MacOSX
  Details: cmake --help-policy CMP0025
- do not declare Clang as GCC compiler

- '-fvisibility' flag was updated
- PVAPI library detection was changed

- 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)

(2.4) Backports from master branch (#7854)

Pdb file support was changed in CMake 2.8.12, support was added in CMake 3.1.0 to work around the problems created by the change introduced in CMake 2.8.12.

(cherry picked from commit b75cbfde)

Conflicts:

	cmake/OpenCVModule.cmake

Odd that it wasn't merged automatically...

Allow to build samples via OpenCV binaries from distribution package
(find_package with OpenCV_DIR).

add INSTALL command for TBB target

Debug library name prefix changed no "_debug" to fit pragmas in TBB
headers.

Downgrade TBB to 4.1u2 for all platforms, except WinRT.

Development release of TBB with WinRT support added;
TBB.dll is placed in bin folder now.

TIFF build was fixed;
Direct Show back-end was enabled;
More accurate guards implemented.

HardFP and SoftFP toolchains joined to one;
RPATH skiping added.

Toolchain for arm hardfp added.