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();
                   ^
}
        entry.pushKV("scriptPubKey", HexStr(scriptPubKey.begin(), scriptPubKey.end()));
        entry.pushKV("amount", ValueFromAmount(out.tx->tx->vout[out.i].nValue));
        entry.pushKV("confirmations", out.nDepth);
        entry.pushKV("spendable", out.fSpendable);
        entry.pushKV("solvable", out.fSolvable);
        if (out.fSolvable) {
            std::unique_ptr<SigningProvider> provider = pwallet->GetSolvingProvider(scriptPubKey);
            if (provider) {
                auto descriptor = InferDescriptor(scriptPubKey, *provider);
                entry.pushKV("desc", descriptor->ToString());
            }
        }
        if (avoid_reuse) entry.pushKV("reused", reused);
        entry.pushKV("safe", out.fSafe);
        results.push_back(entry);
    }
    return results;
}
void FundTransaction(CWallet* const pwallet, CMutableTransaction& tx, CAmount& fee_out, int& change_position, UniValue options)
{
    
    
    pwallet->BlockUntilSyncedToCurrentChain();
    CCoinControl coinControl;
    change_position = -1;
    bool lockUnspents = false;
    UniValue subtractFeeFromOutputs;
    std::set<int> setSubtractFeeFromOutputs;
    if (!options.isNull()) {