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();
                   ^
{
        UniValue obj(UniValue::VOBJ);
        CPubKey pubkey;
        if (provider && provider->GetPubKey(CKeyID(id), pubkey)) {
            obj.pushKV("pubkey", HexStr(pubkey));
        }
        return obj;
    }
    UniValue operator()(const WitnessV0ScriptHash& id) const
    {
        UniValue obj(UniValue::VOBJ);
        CScript subscript;
        CRIPEMD160 hasher;
        uint160 hash;
        hasher.Write(id.begin(), 32).Finalize(hash.begin());
        if (provider && provider->GetCScript(CScriptID(hash), subscript)) {
            ProcessSubScript(subscript, obj);
        }
        return obj;
    }
    UniValue operator()(const WitnessUnknown& id) const { return UniValue(UniValue::VOBJ); }