The wallet code uses `Optional::emplace()`, which was only added in boost 1.56, see https://github.com/boostorg/optional/commit/2e583aaf30964afd66c1284aa71d0d8b42fa440d

So for 0.21.0 there are two options:
* Work around `emplace` and keep the minimum boost version below 1.56
* Require boost 1.56


```
wallet/rpcwallet.cpp: In function ‘UniValue listsinceblock(const JSONRPCRequest&)’:
wallet/rpcwallet.cpp:1560:16: error: ‘Optional<int>’ has no member named ‘emplace’
         height.emplace();
                ^
wallet/rpcwallet.cpp:1561:19: error: ‘Optional<int>’ has no member named ‘emplace’
         altheight.emplace();
                   ^
CPubKey key(solutions_data[i].begin(), solutions_data[i].end());
                pubkeys.push_back(HexStr(key.begin(), key.end()));
            }
            obj.pushKV("pubkeys", std::move(pubkeys));
        }
    }
    explicit DescribeWalletAddressVisitor(const SigningProvider* _provider) : provider(_provider) {}
    UniValue operator()(const CNoDestination& dest) const { return UniValue(UniValue::VOBJ); }
    UniValue operator()(const PKHash& pkhash) const
    {
        CKeyID keyID(pkhash);
        UniValue obj(UniValue::VOBJ);
        CPubKey vchPubKey;
        if (provider && provider->GetPubKey(keyID, vchPubKey)) {
            obj.pushKV("pubkey", HexStr(vchPubKey));
            obj.pushKV("iscompressed", vchPubKey.IsCompressed());
        }
        return obj;
    }
    UniValue operator()(const ScriptHash& scripthash) const
    {
        CScriptID scriptID(scripthash);
        UniValue obj(UniValue::VOBJ);
        CScript subscript;
        if (provider && provider->GetCScript(scriptID, subscript)) {
            ProcessSubScript(subscript, obj);
        }
        return obj;
    }
    UniValue operator()(const WitnessV0KeyHash& id) const