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();
                   ^
RPCExamples{
            HelpExampleCli("getbalances", "") +
            HelpExampleRpc("getbalances", "")},
    }.Check(request);
    
    
    wallet.BlockUntilSyncedToCurrentChain();
    LOCK(wallet.cs_wallet);
    const auto bal = wallet.GetBalance();
    UniValue balances{UniValue::VOBJ};
    {
        UniValue balances_mine{UniValue::VOBJ};
        balances_mine.pushKV("trusted", ValueFromAmount(bal.m_mine_trusted));
        balances_mine.pushKV("untrusted_pending", ValueFromAmount(bal.m_mine_untrusted_pending));
        balances_mine.pushKV("immature", ValueFromAmount(bal.m_mine_immature));
        if (wallet.IsWalletFlagSet(WALLET_FLAG_AVOID_REUSE)) {
            
            
            const auto full_bal = wallet.GetBalance(0, false);
            balances_mine.pushKV("used", ValueFromAmount(full_bal.m_mine_trusted + full_bal.m_mine_untrusted_pending - bal.m_mine_trusted - bal.m_mine_untrusted_pending));
        }
        balances.pushKV("mine", balances_mine);
    }
    auto spk_man = wallet.GetLegacyScriptPubKeyMan();
    if (spk_man && spk_man->HaveWatchOnly()) {
        UniValue balances_watchonly{UniValue::VOBJ};
        balances_watchonly.pushKV("trusted", ValueFromAmount(bal.m_watchonly_trusted));
        balances_watchonly.pushKV("untrusted_pending", ValueFromAmount(bal.m_watchonly_untrusted_pending));
        balances_watchonly.pushKV("immature", ValueFromAmount(bal.m_watchonly_immature));
        balances.pushKV("watchonly", balances_watchonly);