message: Fix build on mac
diff: #ifndef ONNX_ONNX_PB_H
  #define ONNX_ONNX_PB_H
  +/**
  + * Macro for marking functions as having public visibility.
  + * Ported from folly/CPortability.h
  + */
  +#ifndef __GNUC_PREREQ
  +#if defined __GNUC__ && defined __GNUC_MINOR__
  +#define __GNUC_PREREQ(maj, min) \
  + ((__GNUC__ << 16) + __GNUC_MINOR__ >= ((maj) << 16) + (min))
  +#else
  +#define __GNUC_PREREQ(maj, min) 0
  +#endif
  +#endif
  +
  // Defines ONNX_EXPORT and ONNX_IMPORT. On Windows, this corresponds to
  // different declarations (dllexport and dllimport). On Linux/Mac, it just
  // resolves to the same "default visibility" setting.
  