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();
                   ^
}
    return NullUniValue;
}
static CTransactionRef SendMoney(CWallet* const pwallet, const CTxDestination& address, CAmount nValue, bool fSubtractFeeFromAmount, const CCoinControl& coin_control, mapValue_t mapValue)
{
    CAmount curBalance = pwallet->GetBalance(0, coin_control.m_avoid_address_reuse).m_mine_trusted;
    
    if (nValue <= 0)
        throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid amount");
    if (nValue > curBalance)
        throw JSONRPCError(RPC_WALLET_INSUFFICIENT_FUNDS, "Insufficient funds");
    
    CScript scriptPubKey = GetScriptForDestination(address);
    
    CAmount nFeeRequired = 0;
    bilingual_str error;
    std::vector<CRecipient> vecSend;
    int nChangePosRet = -1;
    CRecipient recipient = {scriptPubKey, nValue, fSubtractFeeFromAmount};
    vecSend.push_back(recipient);
    CTransactionRef tx;
    if (!pwallet->CreateTransaction(vecSend, tx, nFeeRequired, nChangePosRet, error, coin_control)) {
        if (!fSubtractFeeFromAmount && nValue + nFeeRequired > curBalance)
            error = strprintf(Untranslated("Error: This transaction requires a transaction fee of at least %s"), FormatMoney(nFeeRequired));
        throw JSONRPCError(RPC_WALLET_ERROR, error.original);
    }
    pwallet->CommitTransaction(tx, std::move(mapValue), {} );
    return tx;
}
static UniValue sendtoaddress(const JSONRPCRequest& request)