signal: wake all condition-variable waiters on shutdown

On SIGINT/SIGTERM, the signal thread only set should_run to 0 and
relied on a wake-up chain to propagate the shutdown: the self-pipe
wakes the softsynth thread out of select(), which on exit signals
runner_awake to wake the espeak thread.

That chain breaks whenever the softsynth thread is not sitting in
select() but waiting on stop_acknowledged in request_espeak_stop():
nobody ever signals that condition variable on shutdown, so the thread
never re-evaluates should_run and the process never exits.

Broadcast all three condition variables after clearing should_run, so
that every parked thread re-checks its predicate, whichever wait it is
blocked in.

Helps: https://github.com/linux-speakup/espeakup/issues/45
Helps: https://github.com/linux-speakup/espeakup/issues/62

Co-Authored-By: Claude <noreply@anthropic.com>

src/signal.c

@@ -58,6 +58,14 @@ void *signal_thread(void *arg)
 		case SIGTERM:
 			pthread_mutex_lock(&queue_guard);
 			should_run = 0;
+			/* Wake up any thread waiting on a condition variable so
+			 * that it notices the shutdown request: the softsynth
+			 * thread may be waiting for a stop acknowledgement, and
+			 * the espeak thread may be waiting for work or throttling
+			 * before a retry. */
+			pthread_cond_broadcast(&runner_awake);
+			pthread_cond_broadcast(&wake_stop);
+			pthread_cond_broadcast(&stop_acknowledged);
 			pthread_mutex_unlock(&queue_guard);
 			break;
 		default: