MHD_start_daemon(): warn if messages could be printed by wrong logger

If MHD_OPTION_EXTERNAL_LOGGER is specified and not used in the first
position then some messages could be printed by standard MHD logger
before this option is processed. Warn about this situation.

src/include/microhttpd.h

@@ -1520,6 +1520,8 @@ enum MHD_OPTION
    * a function of type #MHD_LogCallback and the second a pointer
    * `void *` which will be passed as the first argument to the log
    * callback.
+   * Should be specified as the first option, otherwise some messages
+   * may be printed by standard MHD logger during daemon startup.
    *
    * Note that MHD will not generate any log messages
    * if it was compiled without the "--enable-messages"

src/microhttpd/daemon.c

@@ -5741,6 +5741,12 @@ parse_options_va (struct MHD_Daemon *daemon,
                                          VfprintfFunctionPointerType);
       daemon->custom_error_log_cls = va_arg (ap,
                                              void *);
+      if (1 != daemon->num_opts)
+        MHD_DLOG (daemon,
+                  _ ("MHD_OPTION_EXTERNAL_LOGGER is not the first option "
+                     "specified for the daemon. Some messages may be "
+                     "printed by the standard MHD logger.\n"));
+
 #else
       va_arg (ap,
               VfprintfFunctionPointerType);