Fix signal_handler_run_in_c_thread test (#2701)

ocaml/testsuite/tests/lib-threads/signal_handler_run_in_c_thread_stubs.c

@@ -4,10 +4,14 @@
 
 static pthread_cond_t cond = PTHREAD_COND_INITIALIZER;
 static pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
+static int thread_started = 0;
 
 static void* in_thread(void* unused)
 {
-  (void) pthread_cond_signal(&cond);
+  pthread_mutex_lock(&mutex);
+  thread_started = 1;
+  pthread_cond_signal(&cond);
+  pthread_mutex_unlock(&mutex);
   /* Signal to be received in this thread by the OCaml signal handler */
   while (1);
 }
@@ -16,7 +20,11 @@ value test_signal_handler_run_in_c_thread(value unit)
 {
   pthread_t thread;
   pthread_create(&thread, NULL, &in_thread, NULL);
-  pthread_cond_wait(&cond, &mutex);
+  pthread_mutex_lock(&mutex);
+  while (!thread_started) {
+    pthread_cond_wait(&cond, &mutex);
+  }
+  pthread_mutex_unlock(&mutex);
   pthread_kill(thread, SIGUSR1);
   return Val_unit;
 }