Fix build on mac (onnx#514)

Author: dzhulgakov

onnx/onnx_pb.h

@@ -4,6 +4,19 @@
 #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.