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();
                   ^
"\nList all labels\n"
            + HelpExampleCli("listlabels", "") +
            "\nList labels that have receiving addresses\n"
            + HelpExampleCli("listlabels", "receive") +
            "\nList labels that have sending addresses\n"
            + HelpExampleCli("listlabels", "send") +
            "\nAs a JSON-RPC call\n"
            + HelpExampleRpc("listlabels", "receive")
                },
            }.Check(request);
    LOCK(pwallet->cs_wallet);
    std::string purpose;
    if (!request.params[0].isNull()) {
        purpose = request.params[0].get_str();
    }
    
    std::set<std::string> label_set;
    for (const std::pair<const CTxDestination, CAddressBookData>& entry : pwallet->m_address_book) {
        if (entry.second.IsChange()) continue;
        if (purpose.empty() || entry.second.purpose == purpose) {
            label_set.insert(entry.second.GetLabel());
        }
    }
    UniValue ret(UniValue::VARR);
    for (const std::string& name : label_set) {
        ret.push_back(name);
    }
    return ret;
}
UniValue sethdseed(const JSONRPCRequest& request)