Fix macOS 10.12 build warning

OSMemoryBarrier() has been deprecated as of macOS 10.12. Compile it only if on a version where it’s not deprecated.

Author: droark

port/atomic_pointer.h

@@ -26,8 +26,10 @@
 #include <windows.h>
 #endif
 #ifdef OS_MACOSX
+#if defined(__MAC_OS_X_VERSION_MIN_REQUIRED) && __MAC_OS_X_VERSION_MIN_REQUIRED < 101200
 #include <libkern/OSAtomic.h>
 #endif
+#endif
 
 #if defined(_M_X64) || defined(__x86_64__)
 #define ARCH_CPU_X86_FAMILY 1
@@ -55,10 +57,12 @@ namespace port {
 
 // Mac OS
 #elif defined(OS_MACOSX)
+#if defined(__MAC_OS_X_VERSION_MIN_REQUIRED) && __MAC_OS_X_VERSION_MIN_REQUIRED < 101200
 inline void MemoryBarrier() {
   OSMemoryBarrier();
 }
 #define LEVELDB_HAVE_MEMORY_BARRIER
+#endif
 
 // Gcc on x86
 #elif defined(ARCH_CPU_X86_FAMILY) && defined(__GNUC__)