[build]: check std::system for -[alert|block|wallet]notify

Platforms such as iOs do not support launching a process
through system().

src/wallet/wallet.cpp

@@ -1008,6 +1008,7 @@ bool CWallet::AddToWallet(const CWalletTx& wtxIn, bool fFlushOnClose)
     // Notify UI of new or updated transaction
     NotifyTransactionChanged(this, hash, fInsertedNew ? CT_NEW : CT_UPDATED);
 
+#if defined(HAVE_SYSTEM)
     // notify an external script when a wallet transaction comes in or is updated
     std::string strCmd = gArgs.GetArg("-walletnotify", "");
 
@@ -1017,6 +1018,7 @@ bool CWallet::AddToWallet(const CWalletTx& wtxIn, bool fFlushOnClose)
         std::thread t(runCommand, strCmd);
         t.detach(); // thread runs free
     }
+#endif
 
     return true;
 }