issue #172: flush audio on cancel requests.

This is a partial fix for an audio regression. The other part of
the fix requires a change in pcaudiolib to reopen the audio device
on flush requests for affected audio drivers/devices.

src/libespeak-ng/fifo.c

 			status = a_status;
 
 		if (a_stop_is_required) {
+			// cancel the audio early, to be more responsive when using eSpeak NG
+			// for audio.
+			cancel_audio();
+
 			// acknowledge the stop request
 			if((a_status = pthread_mutex_lock(&my_mutex)) != ENS_OK)
 				return a_status;

src/libespeak-ng/speech.c

 char path_home[N_PATH_HOME]; // this is the espeak-ng-data directory
 extern int saved_parameters[N_SPEECH_PARAM]; // Parameters saved on synthesis start
 
+void cancel_audio(void)
+{
+#ifdef HAVE_PCAUDIOLIB_AUDIO_H
+	if ((my_mode & ENOUTPUT_MODE_SPEAK_AUDIO) == ENOUTPUT_MODE_SPEAK_AUDIO) {
+		audio_object_flush(my_audio);
+	}
+#endif
+}
+
 static int dispatch_audio(short *outbuf, int length, espeak_EVENT *event)
 {
 	int a_wave_can_be_played = 1;

src/libespeak-ng/speech.h

 } MNEM_TAB;
 int LookupMnem(MNEM_TAB *table, const char *string);
 
+void cancel_audio(void);
+
 extern char path_home[N_PATH_HOME];    // this is the espeak-ng-data directory
 
 extern ESPEAK_NG_API void strncpy0(char *to, const char *from, int size);