message: Added support for Clang build hardening (including Apple)
diff: # - https://wiki.debian.org/Hardening
  # - https://wiki.gentoo.org/wiki/Hardened/Toolchain
  # - https://docs.microsoft.com/en-us/cpp/build/reference/sdl-enable-additional-security-checks
  -
  +# - https://developer.apple.com/library/archive/documentation/Security/Conceptual/SecureCodingGuide/Articles/BufferOverflows.html
  set(OPENCV_LINKER_DEFENSES_FLAGS_COMMON "")
  @@ -44,6 +44,12 @@ if(MSVC)
  if(NOT X86_64)
  set(OPENCV_LINKER_DEFENSES_FLAGS_COMMON "${OPENCV_LINKER_DEFENSES_FLAGS_COMMON} /safeseh")
  endif()
  +elseif(CV_CLANG)
  + ocv_add_defense_compiler_flag("-fstack-protector-strong")
  + ocv_add_defense_compiler_flag_release("-D_FORTIFY_SOURCE=2")
  + if (NOT APPLE)
  + set(OPENCV_LINKER_DEFENSES_FLAGS_COMMON "${OPENCV_LINKER_DEFENSES_FLAGS_COMMON} -z noexecstack -z relro -z now" )
  + endif()
  elseif(CV_GCC)
  if(CMAKE_CXX_COMPILER_VERSION VERSION_LESS "4.9")
  ocv_add_defense_compiler_flag("-fstack-protector")
  