Fix libc++16 failures

Similar to https://github.com/boostorg/core/commit/de8fe4fad7a4a843e6d0dfdaa2a4962fa8e0d50f

test/hash_is_avalanching_test.cpp

@@ -35,7 +35,11 @@ int main()
 
 #endif
 
+#if defined(_LIBCPP_VERSION) && _LIBCPP_VERSION >= 160000
+// std::char_traits<Ch> is deprecated for non-char types
+#else
     BOOST_TEST_TRAIT_FALSE(( hash_is_avalanching< boost::hash<std::basic_string<my_char> > > ));
+#endif
 
     return boost::report_errors();
 }