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, "abandoned", "'true' if the transaction has been abandoned (inputs are respendable). Only available for the \n"
                                     "'send' category of transactions."},
                            }},
                        }},
                        {RPCResult::Type::STR_HEX, "hex", "Raw data for transaction"},
                        {RPCResult::Type::OBJ, "decoded", "Optional, the decoded transaction (only present when `verbose` is passed)",
                        {
                            {RPCResult::Type::ELISION, "", "Equivalent to the RPC decoderawtransaction method, or the RPC getrawtransaction method when `verbose` is passed."},
                        }},
                    })
                },
                RPCExamples{
                    HelpExampleCli("gettransaction", "\"1075db55d416d3ca199f55b6084e2115b9345e16c5cf302fc80e9d5fbf5d48d\"")
            + HelpExampleCli("gettransaction", "\"1075db55d416d3ca199f55b6084e2115b9345e16c5cf302fc80e9d5fbf5d48d\" true")
            + HelpExampleCli("gettransaction", "\"1075db55d416d3ca199f55b6084e2115b9345e16c5cf302fc80e9d5fbf5d48d\" false true")
            + HelpExampleRpc("gettransaction", "\"1075db55d416d3ca199f55b6084e2115b9345e16c5cf302fc80e9d5fbf5d48d\"")
                },
            }.Check(request);
    
    
    pwallet->BlockUntilSyncedToCurrentChain();
    LOCK(pwallet->cs_wallet);
    uint256 hash(ParseHashV(request.params[0], "txid"));
    isminefilter filter = ISMINE_SPENDABLE;
    if (ParseIncludeWatchonly(request.params[1], *pwallet)) {
        filter |= ISMINE_WATCH_ONLY;
    }
    bool verbose = request.params[2].isNull() ? false : request.params[2].get_bool();
    UniValue entry(UniValue::VOBJ);
    auto it = pwallet->mapWallet.find(hash);