cmake: avoid amending of CMAKE_COMPILER_IS_[GNUCXX|CLANGCXX|CCACHE] vars

- 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

modules/python/common.cmake

@@ -64,7 +64,7 @@ if(MSVC)
   add_definitions(-DCVAPI_EXPORTS)
 endif()
 
-if(CMAKE_COMPILER_IS_GNUCXX AND NOT ENABLE_NOISY_WARNINGS)
+if((CV_GCC OR CV_CLANG) AND NOT ENABLE_NOISY_WARNINGS)
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wno-unused-function")
 endif()