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();
                   ^
UniValue result(UniValue::VOBJ);
    result.pushKV("hex", EncodeHexTx(CTransaction(tx)));
    result.pushKV("fee", ValueFromAmount(fee));
    result.pushKV("changepos", change_position);
    return result;
}
UniValue signrawtransactionwithwallet(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{"signrawtransactionwithwallet",
                "\nSign inputs for raw transaction (serialized, hex-encoded).\n"
                "The second optional argument (may be null) is an array of previous transaction outputs that\n"
                "this transaction depends on but may not yet be in the block chain." +
        HELP_REQUIRING_PASSPHRASE,
                {
                    {"hexstring", RPCArg::Type::STR, RPCArg::Optional::NO, "The transaction hex string"},
                    {"prevtxs", RPCArg::Type::ARR, RPCArg::Optional::OMITTED_NAMED_ARG, "The previous dependent transaction outputs",
                        {
                            {"", RPCArg::Type::OBJ, RPCArg::Optional::OMITTED, "",
                                {
                                    {"txid", RPCArg::Type::STR_HEX, RPCArg::Optional::NO, "The transaction id"},
                                    {"vout", RPCArg::Type::NUM, RPCArg::Optional::NO, "The output number"},
                                    {"scriptPubKey", RPCArg::Type::STR_HEX, RPCArg::Optional::NO, "script key"},
                                    {"redeemScript", RPCArg::Type::STR_HEX, RPCArg::Optional::OMITTED, "(required for P2SH) redeem script"},
                                    {"witnessScript", RPCArg::Type::STR_HEX, RPCArg::Optional::OMITTED, "(required for P2WSH or P2SH-P2WSH) witness script"},
                                    {"amount", RPCArg::Type::AMOUNT, RPCArg::Optional::OMITTED, "(required for Segwit inputs) the amount spent"},