Always enable the wave* code irrespective of whether USE_ASYNC is defined.

src/libespeak-ng/wave.c

 
 #include "speech.h"
 
-#ifdef USE_ASYNC
-// This source file is only used for asynchronious modes
-
 #include <stdbool.h>
 #include <stdio.h>
 #include <string.h>
 	}
 	ts->tv_nsec = (long int)t_ns;
 }
-
-#endif

src/libespeak-ng/wave_pulse.c

 
 #include "speech.h"
 
-#ifdef USE_ASYNC
-// This source file is only used for asynchronious modes
-
 #include <stdbool.h>
 #include <stdio.h>
 #include <string.h>
 }
 
 #endif
-#endif

src/libespeak-ng/wave_sada.c

 
 #include "speech.h"
 
-#ifdef USE_ASYNC
-// This source file is only used for asynchronious modes
-
 #include <errno.h>
 #include <string.h>
 #include <stropts.h>
 	}
 	ts->tv_nsec = (long int)t_ns;
 }
-
-#endif