build: attempt to avoid some libc++ problems

Seems there are problems compiling mu with XCode 11.6 (see build tests);
apparently because of libc++ being different from libstdc++.

clang++ builds works fine as long as we're using libstdc++.

lib/utils/mu-sexp.hh

@@ -306,7 +306,8 @@ private:
 
         static bool is_prop_list (Seq::const_iterator b, Seq::const_iterator e) {
                 while (b != e) {
-                        if (!is_prop_name(*b))
+                        const Sexp& s{*b};
+                        if (!is_prop_name(s))
                                 return false;
                         if (++b == e)
                                 return false;