testing clang v.

test/optional_xconfig_HACK_TO_LIST_PREDEFINED_MACROS.cpp

@@ -40,6 +40,7 @@ int main()
 #endif
 
   BOOST_TEST_EQ(empty, BOOST_COMP_GNUC);
+  BOOST_TEST_EQ(empty, BOOST_COMP_CLANG);
   BOOST_TEST_EQ(empty, BOOST_LANG_STDCPP);
   BOOST_TEST_EQ(empty, BOOST_LIB_C_GNU);
   BOOST_TEST_EQ(empty, BOOST_LIB_STD_CXX);