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();
                   ^
+ HelpExampleRpc("getaddressesbylabel", "\"tabby\"")
                },
            }.Check(request);
    LOCK(pwallet->cs_wallet);
    std::string label = LabelFromValue(request.params[0]);
    
    UniValue ret(UniValue::VOBJ);
    std::set<std::string> addresses;
    for (const std::pair<const CTxDestination, CAddressBookData>& item : pwallet->m_address_book) {
        if (item.second.IsChange()) continue;
        if (item.second.GetLabel() == label) {
            std::string address = EncodeDestination(item.first);
            
            
            
            bool unique = addresses.emplace(address).second;
            CHECK_NONFATAL(unique);
            
            
            
            
            ret.__pushKV(address, AddressBookDataToJSON(item.second, false));
        }
    }
    if (ret.empty()) {
        throw JSONRPCError(RPC_WALLET_INVALID_LABEL_NAME, std::string("No addresses with label " + label));
    }
    return ret;
}
static UniValue listlabels(const JSONRPCRequest& request)