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();
                   ^
},
                            },
                        },
                    },
                    {"sighashtype", RPCArg::Type::STR,  "ALL", "The signature hash type. Must be one of\n"
            "       \"ALL\"\n"
            "       \"NONE\"\n"
            "       \"SINGLE\"\n"
            "       \"ALL|ANYONECANPAY\"\n"
            "       \"NONE|ANYONECANPAY\"\n"
            "       \"SINGLE|ANYONECANPAY\""},
                },
                RPCResult{
                    RPCResult::Type::OBJ, "", "",
                    {
                        {RPCResult::Type::STR_HEX, "hex", "The hex-encoded raw transaction with signature(s)"},
                        {RPCResult::Type::BOOL, "complete", "If the transaction has a complete set of signatures"},
                        {RPCResult::Type::ARR, "errors", "Script verification errors (if there are any)",
                        {
                            {RPCResult::Type::OBJ, "", "",
                            {
                                {RPCResult::Type::STR_HEX, "txid", "The hash of the referenced, previous transaction"},
                                {RPCResult::Type::NUM, "vout", "The index of the output to spent and used as input"},
                                {RPCResult::Type::STR_HEX, "scriptSig", "The hex-encoded signature script"},
                                {RPCResult::Type::NUM, "sequence", "Script sequence number"},
                                {RPCResult::Type::STR, "error", "Verification or signing error related to the input"},
                            }},
                        }},
                    }
                },