Enable and fix -Wunused-but-set-variable warnings.

configure.ac

 dnl Reference: https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html
 dnl ================================================================
 
-AX_CHECK_COMPILE_FLAG([-Wunused-function],  [CFLAGS="-Wunused-function $CFLAGS"])
-AX_CHECK_COMPILE_FLAG([-Wunused-parameter], [CFLAGS="-Wunused-parameter $CFLAGS"])
-AX_CHECK_COMPILE_FLAG([-Wunused-variable],  [CFLAGS="-Wunused-variable $CFLAGS"])
+AX_CHECK_COMPILE_FLAG([-Wunused-function],         [CFLAGS="-Wunused-function $CFLAGS"])
+AX_CHECK_COMPILE_FLAG([-Wunused-parameter],        [CFLAGS="-Wunused-parameter $CFLAGS"])
+AX_CHECK_COMPILE_FLAG([-Wunused-variable],         [CFLAGS="-Wunused-variable $CFLAGS"])
+AX_CHECK_COMPILE_FLAG([-Wunused-but-set-variable], [CFLAGS="-Wunused-but-set-variable $CFLAGS"])
 
 dnl ================================================================
 dnl Generate output.

src/libespeak-ng/event.c

 	while (1) {
 		int a_stop_is_required = 0;
 
-		int a_status = pthread_mutex_lock(&my_mutex);
+		(void)pthread_mutex_lock(&my_mutex);
 		my_event_is_running = 0;
 
 		while (my_start_is_required == 0) {
 				event->user_data = NULL;
 			}
 
-			a_status = pthread_mutex_lock(&my_mutex);
+			(void)pthread_mutex_lock(&my_mutex);
 			event_delete((espeak_EVENT *)pop());
 			a_stop_is_required = my_stop_is_required;
 			if (a_stop_is_required > 0)
 				my_stop_is_required = 0;
 
-			a_status = pthread_mutex_unlock(&my_mutex);
+			(void)pthread_mutex_unlock(&my_mutex);
 		}
 
-		a_status = pthread_mutex_lock(&my_mutex);
+		(void)pthread_mutex_lock(&my_mutex);
 
 		my_event_is_running = 0;
 
 				my_stop_is_required = 0;
 		}
 
-		a_status = pthread_mutex_unlock(&my_mutex);
+		(void)pthread_mutex_unlock(&my_mutex);
 
 		if (a_stop_is_required > 0) {
 			// no mutex required since the stop command is synchronous
 			init();
 
 			// acknowledge the stop request
-			espeak_ng_STATUS a_status = pthread_mutex_lock(&my_mutex);
+			(void)pthread_mutex_lock(&my_mutex);
 			my_stop_is_acknowledged = 1;
-			a_status = pthread_cond_signal(&my_cond_stop_is_acknowledged);
-			a_status = pthread_mutex_unlock(&my_mutex);
+			(void)pthread_cond_signal(&my_cond_stop_is_acknowledged);
+			(void)pthread_mutex_unlock(&my_mutex);
 		}
 	}