CPATH entries are processed before any -isystem:
https://gcc.gnu.org/onlinedocs/cpp/Environment-Variables.html

- 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

- Optimizations set change. Now IPP integrations will provide code for SSE42, AVX2 and AVX512 (SKX) CPUs only. For HW below SSE42 IPP code is disabled.
- Performance regressions fixes for IPP code paths;
- cv::boxFilter integration improvement;
- cv::filter2D integration improvement;