- remove duplicates
- restore "always on" missing entries
- fix FP16 detection on MSVC

via GCC/Clang __POPCNT__ define

Reproducer:
    cmake -DCPU_BASELINE=AVX2 -DCV_DISABLE_OPTIMIZATION=ON ...

  * remove NVidia and Nvidia
  * replace Cuda with CUDA
  * keep the letters for API

CMake generated files is a part of target depends list.
Details: `cmake --help-policy CMP0058`

* Add CPU default extensions loading in IE dnn backend

* Load cpu_extensions for the future Intel's Inference Engine

- properly detect Android SDK support of Ant/Gradle projects
- divide processing of And/Gradle based projects

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

during opencv_world builds

add_dependencies(${target}) doesn't control regeneration of .apk file,
because "${target}" is dummy target.

When compiling with OpenNI2 flag active, CMake will not be able to find OpenNI.h due to the environment path not set in OpenCVFindOpenNI2.cmake. This PR rectifies this issue.

This suppresses the interpretation of string literals by switching the ocv_download_log from a macro to a function.
This avoid crashes when the string to log contains escape characters.

More details about the problem this PR fixes are available at: https://github.com/opencv/opencv_contrib/issues/1131