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();
                   ^
}
    obj.pushKV("keypoolsize", (int64_t)kpExternalSize);
    LegacyScriptPubKeyMan* spk_man = pwallet->GetLegacyScriptPubKeyMan();
    if (spk_man) {
        CKeyID seed_id = spk_man->GetHDChain().seed_id;
        if (!seed_id.IsNull()) {
            obj.pushKV("hdseedid", seed_id.GetHex());
        }
    }
    if (pwallet->CanSupportFeature(FEATURE_HD_SPLIT)) {
        obj.pushKV("keypoolsize_hd_internal",   (int64_t)(pwallet->GetKeyPoolSize() - kpExternalSize));
    }
    if (pwallet->IsCrypted()) {
        obj.pushKV("unlocked_until", pwallet->nRelockTime);
    }
    obj.pushKV("paytxfee", ValueFromAmount(pwallet->m_pay_tx_fee.GetFeePerK()));
    obj.pushKV("private_keys_enabled", !pwallet->IsWalletFlagSet(WALLET_FLAG_DISABLE_PRIVATE_KEYS));
    obj.pushKV("avoid_reuse", pwallet->IsWalletFlagSet(WALLET_FLAG_AVOID_REUSE));
    if (pwallet->IsScanning()) {
        UniValue scanning(UniValue::VOBJ);
        scanning.pushKV("duration", pwallet->ScanningDuration() / 1000);
        scanning.pushKV("progress", pwallet->ScanningProgress());
        obj.pushKV("scanning", scanning);
    } else {
        obj.pushKV("scanning", false);
    }
    obj.pushKV("descriptors", pwallet->IsWalletFlagSet(WALLET_FLAG_DESCRIPTORS));
    return obj;
}
static UniValue listwalletdir(const JSONRPCRequest& request)