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();
                   ^
{RPCResult::Type::STR, "", ""},
                        }},
                    }
                },
                RPCExamples{
            "\nBump the fee, get the new transaction\'s txid\n" +
                    HelpExampleCli("bumpfee", "<txid>")
                },
            }.Check(request);
    RPCTypeCheck(request.params, {UniValue::VSTR, UniValue::VOBJ});
    uint256 hash(ParseHashV(request.params[0], "txid"));
    CCoinControl coin_control;
    coin_control.fAllowWatchOnly = pwallet->IsWalletFlagSet(WALLET_FLAG_DISABLE_PRIVATE_KEYS);
    
    coin_control.m_signal_bip125_rbf = true;
    if (!request.params[1].isNull()) {
        UniValue options = request.params[1];
        RPCTypeCheckObj(options,
            {
                {"confTarget", UniValueType(UniValue::VNUM)},
                {"fee_rate", UniValueType(UniValue::VNUM)},
                {"replaceable", UniValueType(UniValue::VBOOL)},
                {"estimate_mode", UniValueType(UniValue::VSTR)},
            },
            true, true);
        if (options.exists("confTarget") && options.exists("fee_rate")) {
            throw JSONRPCError(RPC_INVALID_PARAMETER, "confTarget can't be set with fee_rate. Please provide either a confirmation target in blocks for automatic fee estimation, or an explicit fee rate.");
        } else if (options.exists("confTarget")) { 
            coin_control.m_confirm_target = ParseConfirmTarget(options["confTarget"], pwallet->chain().estimateMaxBlocks());
        } else if (options.exists("fee_rate")) {