message: try to fix libc++ build
diff: @@ -1096,7 +1096,7 @@ void ICmp::print(ostream &os) const {
  }
  static StateValue build_icmp_chain(const expr &var,
  - function<StateValue(ICmp::Cond)> &fn,
  + const function<StateValue(ICmp::Cond)> &fn,
  ICmp::Cond cond = ICmp::Any,
  StateValue last = StateValue()) {
  auto old_cond = cond;
  @@ -1159,7 +1159,7 @@ StateValue ICmp::toSMT(State &s) const {
  }
  auto scalar = [&](const StateValue &a, const StateValue &b) -> StateValue {
  - function fn2 = [&](Cond c) { return fn(a.value, b.value, c); };
  + auto fn2 = [&](Cond c) { return fn(a.value, b.value, c); };
  auto v = cond != Any ? fn2(cond) : build_icmp_chain(cond_var(), fn2);
  return { v.value.toBVBool(), a.non_poison && b.non_poison && v.non_poison };
  };
  