Remove 'boost::optional'-related gcc warnings

Summary: Backport of core [[bitcoin/bitcoin#15292 | PR15292]].

Test Plan:
With GCC:
  ninja check
Make sure the warnings are gone (tested with GCC 7.5).

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5696

src/optional.h

@@ -5,11 +5,18 @@
 #ifndef BITCOIN_OPTIONAL_H
 #define BITCOIN_OPTIONAL_H
 
+#include <utility>
+
 #include <boost/optional.hpp>
 
 //! Substitute for C++17 std::optional
 template <typename T> using Optional = boost::optional<T>;
 
+//! Substitute for C++17 std::make_optional
+template <typename T> Optional<T> MakeOptional(bool condition, T &&value) {
+    return boost::make_optional(condition, std::forward<T>(value));
+}
+
 //! Substitute for C++17 std::nullopt
 static auto &nullopt = boost::none;
 

src/wallet/rpcwallet.cpp

@@ -1939,9 +1939,11 @@ static UniValue listsinceblock(const Config &config,
     auto locked_chain = pwallet->chain().lock();
     LOCK(pwallet->cs_wallet);
 
-    // Height of the specified block or the common ancestor, if the block
-    // provided was in a deactivated chain.
-    Optional<int> height;
+    // The way the 'height' is initialized is just a workaround for the gcc bug
+    // #47679 since version 4.6.0. Height of the specified block or the common
+    // ancestor, if the block provided was in a deactivated chain.
+    Optional<int> height = MakeOptional(false, int());
+
     // Height of the specified block, even if it's in a deactivated chain.
     Optional<int> altheight;
     int target_confirms = 1;

src/wallet/wallet.cpp

@@ -1851,7 +1851,9 @@ CWallet::ScanResult CWallet::ScanForWalletTransactions(
         ShowProgress(strprintf("%s " + _("Rescanning..."), GetDisplayName()),
                      0);
         BlockHash tip_hash;
-        Optional<int> block_height;
+        // The way the 'block_height' is initialized is just a workaround for
+        // the gcc bug #47679 since version 4.6.0.
+        Optional<int> block_height = MakeOptional(false, int());
         double progress_begin;
         double progress_end;
         {