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, "name", "The wallet name if loaded successfully."},
                        {RPCResult::Type::STR, "warning", "Warning message if wallet was not loaded cleanly."},
                    }
                },
                RPCExamples{
                    HelpExampleCli("loadwallet", "\"test.dat\"")
            + HelpExampleRpc("loadwallet", "\"test.dat\"")
                },
            }.Check(request);
    WalletLocation location(request.params[0].get_str());
    if (!location.Exists()) {
        throw JSONRPCError(RPC_WALLET_NOT_FOUND, "Wallet " + location.GetName() + " not found.");
    } else if (fs::is_directory(location.GetPath())) {
        
        fs::path wallet_dat_file = location.GetPath() / "wallet.dat";
        if (fs::symlink_status(wallet_dat_file).type() == fs::file_not_found) {
            throw JSONRPCError(RPC_WALLET_NOT_FOUND, "Directory " + location.GetName() + " does not contain a wallet.dat file.");
        }
    }
    bilingual_str error;
    std::vector<bilingual_str> warnings;
    std::shared_ptr<CWallet> const wallet = LoadWallet(*g_rpc_chain, location, error, warnings);
    if (!wallet) throw JSONRPCError(RPC_WALLET_ERROR, error.original);
    UniValue obj(UniValue::VOBJ);
    obj.pushKV("name", wallet->GetName());
    obj.pushKV("warning", Join(warnings, Untranslated("\n")).original);
    return obj;
}
static UniValue setwalletflag(const JSONRPCRequest& request)