Define RAPIDJSON_HAS_CXX11_RVALUE_REFS directly in clang

This makes no difference except that it avoids "warning: macro expansion
producing 'defined' has undefined behavior" messages.

include/rapidjson/rapidjson.h

@@ -529,8 +529,12 @@ RAPIDJSON_NAMESPACE_END
 
 #ifndef RAPIDJSON_HAS_CXX11_RVALUE_REFS
 #if defined(__clang__)
-#define RAPIDJSON_HAS_CXX11_RVALUE_REFS __has_feature(cxx_rvalue_references) && \
+#if __has_feature(cxx_rvalue_references) && \
     (defined(_LIBCPP_VERSION) || defined(__GLIBCXX__) && __GLIBCXX__ >= 20080306)
+#define RAPIDJSON_HAS_CXX11_RVALUE_REFS 1
+#else
+#define RAPIDJSON_HAS_CXX11_RVALUE_REFS 0
+#endif
 #elif (defined(RAPIDJSON_GNUC) && (RAPIDJSON_GNUC >= RAPIDJSON_VERSION_CODE(4,3,0)) && defined(__GXX_EXPERIMENTAL_CXX0X__)) || \
       (defined(_MSC_VER) && _MSC_VER >= 1600)