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();
                   ^
throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "Block not found");
        }
    }
    if (!request.params[1].isNull()) {
        target_confirms = request.params[1].get_int();
        if (target_confirms < 1) {
            throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid parameter");
        }
    }
    if (ParseIncludeWatchonly(request.params[2], *pwallet)) {
        filter |= ISMINE_WATCH_ONLY;
    }
    bool include_removed = (request.params[3].isNull() || request.params[3].get_bool());
    int depth = height ? pwallet->GetLastBlockHeight() + 1 - *height : -1;
    UniValue transactions(UniValue::VARR);
    for (const std::pair<const uint256, CWalletTx>& pairWtx : pwallet->mapWallet) {
        const CWalletTx& tx = pairWtx.second;
        if (depth == -1 || abs(tx.GetDepthInMainChain()) < depth) {
            ListTransactions(pwallet, tx, 0, true, transactions, filter, nullptr );
        }
    }
    
    
    UniValue removed(UniValue::VARR);
    while (include_removed && altheight && *altheight > *height) {
        CBlock block;
        if (!pwallet->chain().findBlock(blockId, FoundBlock().data(block)) || block.IsNull()) {
            throw JSONRPCError(RPC_INTERNAL_ERROR, "Can't read block from disk");
        }
        for (const CTransactionRef& tx : block.vtx) {