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();
                   ^
"A locked transaction output will not be chosen by automatic coin selection, when spending bitcoins.\n"
                "Locks are stored in memory only. Nodes start with zero locked outputs, and the locked output list\n"
                "is always cleared (by virtue of process exit) when a node stops or fails.\n"
                "Also see the listunspent call\n",
                {
                    {"unlock", RPCArg::Type::BOOL, RPCArg::Optional::NO, "Whether to unlock (true) or lock (false) the specified transactions"},
                    {"transactions", RPCArg::Type::ARR,  "empty array", "The transaction outputs and within each, the txid (string) vout (numeric).",
                        {
                            {"", RPCArg::Type::OBJ, RPCArg::Optional::OMITTED, "",
                                {
                                    {"txid", RPCArg::Type::STR_HEX, RPCArg::Optional::NO, "The transaction id"},
                                    {"vout", RPCArg::Type::NUM, RPCArg::Optional::NO, "The output number"},
                                },
                            },
                        },
                    },
                },
                RPCResult{
                    RPCResult::Type::BOOL, "", "Whether the command was successful or not"
                },
                RPCExamples{
            "\nList the unspent transactions\n"
            + HelpExampleCli("listunspent", "") +
            "\nLock an unspent transaction\n"
            + HelpExampleCli("lockunspent", "false \"[{\\\"txid\\\":\\\"a08e6907dbbd3d809776dbfc5d82e371b764ed838b5655e72f463568df1aadf0\\\",\\\"vout\\\":1}]\"") +
            "\nList the locked transactions\n"
            + HelpExampleCli("listlockunspent", "") +
            "\nUnlock the transaction again\n"
            + HelpExampleCli("lockunspent", "true \"[{\\\"txid\\\":\\\"a08e6907dbbd3d809776dbfc5d82e371b764ed838b5655e72f463568df1aadf0\\\",\\\"vout\\\":1}]\"") +
            "\nAs a JSON-RPC call\n"