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{
                    RPCResult::Type::STR_AMOUNT, "amount", "The total amount in " + CURRENCY_UNIT + " received for this wallet."
                },
                RPCExamples{
            "\nThe total amount in the wallet with 0 or more confirmations\n"
            + HelpExampleCli("getbalance", "") +
            "\nThe total amount in the wallet with at least 6 confirmations\n"
            + HelpExampleCli("getbalance", "\"*\" 6") +
            "\nAs a JSON-RPC call\n"
            + HelpExampleRpc("getbalance", "\"*\", 6")
                },
            }.Check(request);
    
    
    pwallet->BlockUntilSyncedToCurrentChain();
    LOCK(pwallet->cs_wallet);
    const UniValue& dummy_value = request.params[0];
    if (!dummy_value.isNull() && dummy_value.get_str() != "*") {
        throw JSONRPCError(RPC_METHOD_DEPRECATED, "dummy first argument must be excluded or set to \"*\".");
    }
    int min_depth = 0;
    if (!request.params[1].isNull()) {
        min_depth = request.params[1].get_int();
    }
    bool include_watchonly = ParseIncludeWatchonly(request.params[2], *pwallet);
    bool avoid_reuse = GetAvoidReuseFlag(pwallet, request.params[3]);
    const auto bal = pwallet->GetBalance(min_depth, avoid_reuse);
    return ValueFromAmount(bal.m_mine_trusted + (include_watchonly ? bal.m_watchonly_trusted : 0));
}
static UniValue getunconfirmedbalance(const JSONRPCRequest &request)