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();
                   ^
throw JSONRPCError(RPC_INVALID_PARAMETER, "passphrase can not be empty");
        }
        if (!pwallet->Unlock(strWalletPass)) {
            throw JSONRPCError(RPC_WALLET_PASSPHRASE_INCORRECT, "Error: The wallet passphrase entered was incorrect.");
        }
        pwallet->TopUpKeyPool();
        pwallet->nRelockTime = GetTime() + nSleepTime;
        relock_time = pwallet->nRelockTime;
    }
    
    
    
    
    AssertLockNotHeld(wallet->cs_wallet);
    
    
    
    std::weak_ptr<CWallet> weak_wallet = wallet;
    pwallet->chain().rpcRunLater(strprintf("lockwallet(%s)", pwallet->GetName()), [weak_wallet, relock_time] {
        if (auto shared_wallet = weak_wallet.lock()) {
            LOCK(shared_wallet->cs_wallet);
            
            if (shared_wallet->nRelockTime != relock_time) return;
            shared_wallet->Lock();
            shared_wallet->nRelockTime = 0;
        }
    }, nSleepTime);
    return NullUniValue;
}
static UniValue walletpassphrasechange(const JSONRPCRequest& request)