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();
                   ^
}},
                    }
                },
                RPCExamples{
                    HelpExampleCli("getaddressinfo", "\"" + EXAMPLE_ADDRESS[0] + "\"") +
                    HelpExampleRpc("getaddressinfo", "\"" + EXAMPLE_ADDRESS[0] + "\"")
                },
            }.Check(request);
    LOCK(pwallet->cs_wallet);
    UniValue ret(UniValue::VOBJ);
    CTxDestination dest = DecodeDestination(request.params[0].get_str());
    
    if (!IsValidDestination(dest)) {
        throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "Invalid address");
    }
    std::string currentAddress = EncodeDestination(dest);
    ret.pushKV("address", currentAddress);
    CScript scriptPubKey = GetScriptForDestination(dest);
    ret.pushKV("scriptPubKey", HexStr(scriptPubKey.begin(), scriptPubKey.end()));
    std::unique_ptr<SigningProvider> provider = pwallet->GetSolvingProvider(scriptPubKey);
    isminetype mine = pwallet->IsMine(dest);
    ret.pushKV("ismine", bool(mine & ISMINE_SPENDABLE));
    bool solvable = provider && IsSolvable(*provider, scriptPubKey);
    ret.pushKV("solvable", solvable);
    if (solvable) {
       ret.pushKV("desc", InferDescriptor(scriptPubKey, *provider)->ToString());
    }
    ret.pushKV("iswatchonly", bool(mine & ISMINE_WATCH_ONLY));
    UniValue detail = DescribeWalletAddress(pwallet, dest);
    ret.pushKVs(detail);