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();
                   ^
"\nAs a JSON-RPC call\n"
            + HelpExampleRpc("getreceivedbyaddress", "\"" + EXAMPLE_ADDRESS[0] + "\", 6")
                },
            }.Check(request);
    
    
    pwallet->BlockUntilSyncedToCurrentChain();
    LOCK(pwallet->cs_wallet);
    return ValueFromAmount(GetReceived(*pwallet, request.params,  false));
}
static UniValue getreceivedbylabel(const JSONRPCRequest& request)
{
    std::shared_ptr<CWallet> const wallet = GetWalletForJSONRPCRequest(request);
    const CWallet* const pwallet = wallet.get();
    if (!EnsureWalletIsAvailable(pwallet, request.fHelp)) {
        return NullUniValue;
    }
            RPCHelpMan{"getreceivedbylabel",
                "\nReturns the total amount received by addresses with <label> in transactions with at least [minconf] confirmations.\n",
                {
                    {"label", RPCArg::Type::STR, RPCArg::Optional::NO, "The selected label, may be the default label using \"\"."},
                    {"minconf", RPCArg::Type::NUM,  "1", "Only include transactions confirmed at least this many times."},
                },
                RPCResult{
                    RPCResult::Type::STR_AMOUNT, "amount", "The total amount in " + CURRENCY_UNIT + " received for this label."
                },
                RPCExamples{
            "\nAmount received by the default label with at least 1 confirmation\n"
            + HelpExampleCli("getreceivedbylabel", "\"\"") +
            "\nAmount received at the tabby label including unconfirmed amounts with zero confirmations\n"