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();
                   ^
const auto* address_book_entry = pwallet->FindAddressBookEntry(dest);
    if (pwallet->chain().rpcEnableDeprecated("label") && address_book_entry) {
        ret.pushKV("label", address_book_entry->GetLabel());
    }
    ret.pushKV("ischange", pwallet->IsChange(scriptPubKey));
    ScriptPubKeyMan* spk_man = pwallet->GetScriptPubKeyMan(scriptPubKey);
    if (spk_man) {
        if (const std::unique_ptr<CKeyMetadata> meta = spk_man->GetMetadata(dest)) {
            ret.pushKV("timestamp", meta->nCreateTime);
            if (meta->has_key_origin) {
                ret.pushKV("hdkeypath", WriteHDKeypath(meta->key_origin.path));
                ret.pushKV("hdseedid", meta->hd_seed_id.GetHex());
                ret.pushKV("hdmasterfingerprint", HexStr(meta->key_origin.fingerprint, meta->key_origin.fingerprint + 4));
            }
        }
    }
    
    
    
    
    
    UniValue labels(UniValue::VARR);
    if (address_book_entry) {
        
        
        if (pwallet->chain().rpcEnableDeprecated("labelspurpose")) {
            labels.push_back(AddressBookDataToJSON(*address_book_entry, true));