[RWMutex] Simplify availability check
Check for the actual version number for the scenarios where the macOS
version isn't available (__MAC_10_12).
llvm-svn: 369154
diff --git a/llvm/lib/Support/RWMutex.cpp b/llvm/lib/Support/RWMutex.cpp
index f89e0ed..5accf73 100644
--- a/llvm/lib/Support/RWMutex.cpp
+++ b/llvm/lib/Support/RWMutex.cpp
@@ -14,7 +14,7 @@
#include "llvm/Support/RWMutex.h"
#include "llvm/Config/config.h"
-#if defined(USE_RW_MUTEX_IMPL)
+#if defined(LLVM_USE_RW_MUTEX_IMPL)
using namespace llvm;
using namespace sys;