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();
                   ^
}
        const COutPoint outpt(txid, nOutput);
        const auto it = pwallet->mapWallet.find(outpt.hash);
        if (it == pwallet->mapWallet.end()) {
            throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid parameter, unknown transaction");
        }
        const CWalletTx& trans = it->second;
        if (outpt.n >= trans.tx->vout.size()) {
            throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid parameter, vout index out of bounds");
        }
        if (pwallet->IsSpent(outpt.hash, outpt.n)) {
            throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid parameter, expected unspent output");
        }
        const bool is_locked = pwallet->IsLockedCoin(outpt.hash, outpt.n);
        if (fUnlock && !is_locked) {
            throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid parameter, expected locked output");
        }
        if (!fUnlock && is_locked) {
            throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid parameter, output already locked");
        }
        outputs.push_back(outpt);
    }
    
    for (const COutPoint& outpt : outputs) {
        if (fUnlock) pwallet->UnlockCoin(outpt);
        else pwallet->LockCoin(outpt);
    }
    return true;
}
static UniValue listlockunspent(const JSONRPCRequest& request)