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();
                   ^
#include <amount.h>
#include <core_io.h>
#include <interfaces/chain.h>
#include <key_io.h>
#include <node/context.h>
#include <outputtype.h>
#include <policy/feerate.h>
#include <policy/fees.h>
#include <policy/rbf.h>
#include <rpc/rawtransaction_util.h>
#include <rpc/server.h>
#include <rpc/util.h>
#include <script/descriptor.h>
#include <script/sign.h>
#include <util/bip32.h>
#include <util/fees.h>
#include <util/message.h>