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 entry(UniValue::VOBJ);
            if (involvesWatchonly || (pwallet->IsMine(r.destination) & ISMINE_WATCH_ONLY)) {
                entry.pushKV("involvesWatchonly", true);
            }
            MaybePushAddress(entry, r.destination);
            if (wtx.IsCoinBase())
            {
                if (wtx.GetDepthInMainChain() < 1)
                    entry.pushKV("category", "orphan");
                else if (wtx.IsImmatureCoinBase())
                    entry.pushKV("category", "immature");
                else
                    entry.pushKV("category", "generate");
            }
            else
            {
                entry.pushKV("category", "receive");
            }
            entry.pushKV("amount", ValueFromAmount(r.amount));
            if (address_book_entry) {
                entry.pushKV("label", label);
            }
            entry.pushKV("vout", r.vout);
            if (fLong)
                WalletTxToJSON(pwallet->chain(), wtx, entry);
            ret.push_back(entry);
        }
    }
}
static const std::vector<RPCResult> TransactionDescriptionString()