Compile error on Windows: Signal names are undefined. Fix by hiding the whole section.

mysys/my_thr_init.c:
  Compile error on Windows: Both "SIGALRM" and "SIGUSR1" are undefined.
  Fix by hiding the whole section, according to Jani it is not needed on Windows.

mysys/my_thr_init.c

@@ -60,11 +60,14 @@ static uint get_thread_lib(void);
 
 my_bool my_thread_global_init(void)
 {
+#if defined(SIGALRM) || defined(SIGUSR1)
+  /* On Windows, these signals are not defined, but this whole part is not needed. */
   thd_lib_detected= get_thread_lib();
   if (thd_lib_detected == THD_LIB_LT)
     thr_client_alarm= SIGALRM;
   else
     thr_client_alarm= SIGUSR1;
+#endif
 
   if (pthread_key_create(&THR_KEY_mysys,0))
   {