src: Only use TR1 type_traits on OSX<10.9

Mac OSX 10.9 has switched to using libc++ by default.  libc++
provides a C++11 <type_traits> implementation, so we only need
to use the TR1 version when targetting OSX 10.8 or 10.7.

src/util.h

@@ -11,7 +11,12 @@
 #include <stdio.h>
 #include <stdlib.h>
 
-#ifdef __APPLE__
+// OSX 10.9 defaults to libc++ which provides a C++11 <type_traits> header.
+#if defined(__APPLE__) && __ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__ < 1090
+#define USE_TR1_TYPE_TRAITS
+#endif
+
+#ifdef USE_TR1_TYPE_TRAITS
 #include <tr1/type_traits>  // NOLINT(build/c++tr1)
 #else
 #include <type_traits>  // std::remove_reference
@@ -31,7 +36,7 @@ NO_RETURN void Abort();
 NO_RETURN void Assert(const char* const (*args)[4]);
 void DumpBacktrace(FILE* fp);
 
-#ifdef __APPLE__
+#ifdef USE_TR1_TYPE_TRAITS
 template <typename T> using remove_reference = std::tr1::remove_reference<T>;
 #else
 template <typename T> using remove_reference = std::remove_reference<T>;