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();
                   ^
{
    if (include_watchonly.isNull()) {
        
        return pwallet.IsWalletFlagSet(WALLET_FLAG_DISABLE_PRIVATE_KEYS);
    }
    
    return include_watchonly.get_bool();
}
bool HaveKey(const SigningProvider& wallet, const CKey& key)
{
    CKey key2;
    key2.Set(key.begin(), key.end(), !key.IsCompressed());
    return wallet.HaveKey(key.GetPubKey().GetID()) || wallet.HaveKey(key2.GetPubKey().GetID());
}
bool GetWalletNameFromJSONRPCRequest(const JSONRPCRequest& request, std::string& wallet_name)
{
    if (URL_DECODE && request.URI.substr(0, WALLET_ENDPOINT_BASE.size()) == WALLET_ENDPOINT_BASE) {
        
        wallet_name = URL_DECODE(request.URI.substr(WALLET_ENDPOINT_BASE.size()));
        return true;
    }
    return false;
}
std::shared_ptr<CWallet> GetWalletForJSONRPCRequest(const JSONRPCRequest& request)