Bypass OSMemoryBarrier() warning on Mac.

This is a stopgap for removing warnings on Mac builds, so -Werror can be
turned on. C++11 will be required in the nearby future, which guarantees
<atomic> support. Once that happens, the simplified version of this will
match #503

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=188553251

port/atomic_pointer.h

@@ -21,13 +21,12 @@
 #include <stdint.h>
 #ifdef LEVELDB_ATOMIC_PRESENT
 #include <atomic>
+#elif defined(__APPLE__)
+#include <libkern/OSAtomic.h>
 #endif
 #ifdef OS_WIN
 #include <windows.h>
 #endif
-#ifdef __APPLE__
-#include <libkern/OSAtomic.h>
-#endif
 
 #if defined(_M_X64) || defined(__x86_64__)
 #define ARCH_CPU_X86_FAMILY 1
@@ -56,7 +55,11 @@ namespace port {
 // Mac OS
 #elif defined(__APPLE__)
 inline void MemoryBarrier() {
+#if defined(LEVELDB_ATOMIC_PRESENT)
+  std::atomic_thread_fence(std::memory_order_seq_cst);
+#else
   OSMemoryBarrier();
+#endif  // defined(LEVELDB_ATOMIC_PRESENT)
 }
 #define LEVELDB_HAVE_MEMORY_BARRIER