Optionally call printf on LOG_MESSAGE() arguments

With an intentional mistake:

../src/login/logind-dbus.c: In function ‘bus_manager_log_shutdown’:
../src/login/logind-dbus.c:1542:39: error: format ‘%s’ expects a matching ‘char *’ argument [-Werror=format=]
 1542 |                           LOG_MESSAGE("%s %s", message),
      |                                       ^~~~~~~

src/basic/log.h

@@ -300,7 +300,13 @@ bool log_on_console(void) _pure_;
 
 /* Helper to wrap the main message in structured logging. The macro doesn't do much,
  * except to provide visual grouping of the format string and its arguments. */
-#define LOG_MESSAGE(fmt, ...) "MESSAGE=" fmt, ##__VA_ARGS__
+#if LOG_MESSAGE_VERIFICATION
+/* Do a fake formatting of the message string to let the scanner verify the arguments
+ * against the format message. */
+#  define LOG_MESSAGE(fmt, ...) "MESSAGE=%.0d" fmt, printf(fmt, ##__VA_ARGS__), ##__VA_ARGS__
+#else
+#  define LOG_MESSAGE(fmt, ...) "MESSAGE=" fmt, ##__VA_ARGS__
+#endif
 
 void log_received_signal(int level, const struct signalfd_siginfo *si);