message: build_system: ignore old style casts on macOS
  Otherwise, strange enough, we get a warning for tinyxml2.h even though
  the include should be flagged as SYSTEM.
diff: @@ -31,9 +31,11 @@ else()
  endif()
  endif()
  -# We need a define if on APPLE
  if(APPLE)
  + # We need a define if on APPLE
  add_definitions("-DAPPLE")
  + # Otherwise tinyxml2 complains.
  + set(warnings "-Wno-old-style-cast")
  endif()
  # Add DEBUG define for Debug target
  