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();
                   ^
{RPCResult::Type::BOOL, "avoid_reuse", "whether this wallet tracks clean/dirty coins in terms of reuse"},
                        {RPCResult::Type::OBJ, "scanning", "current scanning details, or false if no scan is in progress",
                        {
                            {RPCResult::Type::NUM, "duration", "elapsed seconds since scan start"},
                            {RPCResult::Type::NUM, "progress", "scanning progress percentage [0.0, 1.0]"},
                        }},
                        {RPCResult::Type::BOOL, "descriptors", "whether this wallet uses descriptors for scriptPubKey management"},
                    }},
                },
                RPCExamples{
                    HelpExampleCli("getwalletinfo", "")
            + HelpExampleRpc("getwalletinfo", "")
                },
    }.Check(request);
    
    
    pwallet->BlockUntilSyncedToCurrentChain();
    LOCK(pwallet->cs_wallet);
    UniValue obj(UniValue::VOBJ);
    size_t kpExternalSize = pwallet->KeypoolCountExternalKeys();
    const auto bal = pwallet->GetBalance();
    int64_t kp_oldest = pwallet->GetOldestKeyPoolTime();
    obj.pushKV("walletname", pwallet->GetName());
    obj.pushKV("walletversion", pwallet->GetVersion());
    obj.pushKV("balance", ValueFromAmount(bal.m_mine_trusted));
    obj.pushKV("unconfirmed_balance", ValueFromAmount(bal.m_mine_untrusted_pending));
    obj.pushKV("immature_balance", ValueFromAmount(bal.m_mine_immature));
    obj.pushKV("txcount",       (int)pwallet->mapWallet.size());
    if (kp_oldest > 0) {
        obj.pushKV("keypoololdest", kp_oldest);