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();
                   ^
};
static UniValue DescribeWalletAddress(const CWallet* const pwallet, const CTxDestination& dest)
{
    UniValue ret(UniValue::VOBJ);
    UniValue detail = DescribeAddress(dest);
    CScript script = GetScriptForDestination(dest);
    std::unique_ptr<SigningProvider> provider = nullptr;
    if (pwallet) {
        provider = pwallet->GetSolvingProvider(script);
    }
    ret.pushKVs(detail);
    ret.pushKVs(boost::apply_visitor(DescribeWalletAddressVisitor(provider.get()), dest));
    return ret;
}
static UniValue AddressBookDataToJSON(const CAddressBookData& data, const bool verbose)
{
    UniValue ret(UniValue::VOBJ);
    if (verbose) {
        ret.pushKV("name", data.GetLabel());
    }
    ret.pushKV("purpose", data.purpose);
    return ret;
}
UniValue getaddressinfo(const JSONRPCRequest& request)