build: reject broken roaraudio sndio emulation

roaraudio has some sort of sndio emulation, but apparently its header
file is either blatantly broken, or an old version from the past. The
sio_onvol() function has the wrong return type (void instead of int),
and the SIO_DEVANY symbol is missing entirely. This broke the build,
because the configure check was successful anyway.

wscript

@@ -425,7 +425,7 @@ audio_output_features = [
         'name': '--sndio',
         'desc': 'sndio audio input/output',
         'func': check_statement('sndio.h',
-            'struct sio_par par; sio_initpar(&par)', lib='sndio')
+            'struct sio_par par; sio_initpar(&par); const char *s = SIO_DEVANY', lib='sndio')
     }, {
         'name': '--pulse',
         'desc': 'PulseAudio audio output',