message: add an ONNX_NO_WERROR flag to disable Werror
diff: @@ -316,8 +316,10 @@ if (MSVC)
  STATIC_LIBRARY_FLAGS "${onnx_static_library_flags}")
  elseif(APPLE)
  else()
  + if (NOT ${ONNX_NO_WERROR})
  target_compile_options(onnx PRIVATE -Werror=sign-compare -Werror=conversion)
  endif()
  +endif()
  if(APPLE)
  set_target_properties(onnx PROPERTIES LINK_FLAGS "-undefined dynamic_lookup")
  