Update _GNUC_VER to use the correct implementation of atomic operation

on Mac.

src/google/protobuf/stubs/atomicops.h

@@ -196,6 +196,7 @@ Atomic64 Release_Load(volatile const Atomic64* ptr);
 
 // Apple.
 #elif defined(GOOGLE_PROTOBUF_OS_APPLE)
+#pragma message(_GNUC_VER)
 #if __has_feature(cxx_atomic) || _GNUC_VER >= 407
 #include <google/protobuf/stubs/atomicops_internals_generic_c11_atomic.h>
 #else  // __has_feature(cxx_atomic) || _GNUC_VER >= 407