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();
                   ^
{
public:
    const SigningProvider * const provider;
    void ProcessSubScript(const CScript& subscript, UniValue& obj) const
    {
        
        std::vector<std::vector<unsigned char>> solutions_data;
        txnouttype which_type = Solver(subscript, solutions_data);
        obj.pushKV("script", GetTxnOutputType(which_type));
        obj.pushKV("hex", HexStr(subscript.begin(), subscript.end()));
        CTxDestination embedded;
        if (ExtractDestination(subscript, embedded)) {
            
            UniValue subobj(UniValue::VOBJ);
            UniValue detail = DescribeAddress(embedded);
            subobj.pushKVs(detail);
            UniValue wallet_detail = boost::apply_visitor(*this, embedded);
            subobj.pushKVs(wallet_detail);
            subobj.pushKV("address", EncodeDestination(embedded));
            subobj.pushKV("scriptPubKey", HexStr(subscript.begin(), subscript.end()));
            
            if (subobj.exists("pubkey")) obj.pushKV("pubkey", subobj["pubkey"]);
            obj.pushKV("embedded", std::move(subobj));
        } else if (which_type == TX_MULTISIG) {
            
            
            obj.pushKV("sigsrequired", solutions_data[0][0]);
            UniValue pubkeys(UniValue::VARR);
            for (size_t i = 1; i < solutions_data.size() - 1; ++i) {