message: Enable cmake CMP0077 policy
  When building OpenCV as a sub-project using cmake's add_subdirectory()
  the OpenCV's build options would be overwritten to its default
  state. With cmake 3.13+ the CMP0077 policy, option() honors previous
  definitions via set().
diff: @@ -68,6 +68,9 @@ if(POLICY CMP0075)
  cmake_policy(SET CMP0075 NEW) # CMake 3.12+: Include file check macros honor `CMAKE_REQUIRED_LIBRARIES`
  endif()
  +if(POLICY CMP0077)
  + cmake_policy(SET CMP0077 NEW) # CMake 3.13+: option() honors normal variables.
  +endif()
  #
  # Configure OpenCV CMake hooks
  