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();
                   ^
}
    SecureString passphrase;
    passphrase.reserve(100);
    std::vector<bilingual_str> warnings;
    if (!request.params[3].isNull()) {
        passphrase = request.params[3].get_str().c_str();
        if (passphrase.empty()) {
            
            warnings.emplace_back(Untranslated("Empty string given as passphrase, wallet will not be encrypted."));
        }
    }
    if (!request.params[4].isNull() && request.params[4].get_bool()) {
        flags |= WALLET_FLAG_AVOID_REUSE;
    }
    if (!request.params[5].isNull() && request.params[5].get_bool()) {
        flags |= WALLET_FLAG_DESCRIPTORS;
    }
    bilingual_str error;
    std::shared_ptr<CWallet> wallet;
    WalletCreationStatus status = CreateWallet(*g_rpc_chain, passphrase, flags, request.params[0].get_str(), error, warnings, wallet);
    switch (status) {
        case WalletCreationStatus::CREATION_FAILED:
            throw JSONRPCError(RPC_WALLET_ERROR, error.original);
        case WalletCreationStatus::ENCRYPTION_FAILED:
            throw JSONRPCError(RPC_WALLET_ENCRYPTION_FAILED, error.original);
        case WalletCreationStatus::SUCCESS:
            break;
        
    }
    UniValue obj(UniValue::VOBJ);