meson: temporarily add check to conditionally enable the fade functionality

and in case it's missing, also #define some things to make compilation work,
since this is only available in a pipewire branch right now

modules/module-pipewire/audio-softdsp-endpoint/convert.c

@@ -133,7 +133,12 @@ on_audio_convert_core_done (WpCore *core, GAsyncResult *res,
       &self->format);
 
   /* Only enable control port for input streams */
-  control = direction == PW_DIRECTION_INPUT;
+  control =
+#if defined(HAVE_AUDIOFADE)
+    (direction == PW_DIRECTION_INPUT);
+#else
+    FALSE;
+#endif
 
   /* Configure audioconvert to be both merger and splitter; this means it will
      have an equal number of input and output ports and just passthrough the

modules/module-pipewire/audio-softdsp-endpoint/stream.c

@@ -16,6 +16,14 @@
 
 #include "stream.h"
 
+#if !defined(HAVE_AUDIOFADE)
+# define SPA_PROP_audioFadeDuration 0x30001
+# define SPA_PROP_audioFadeStep 0x30002
+# define SPA_PROP_audioFadeDirection 0x30003
+# define SPA_PROP_audioFadeType 0x30004
+# define SPA_NAME_CONTROL_AUDIO_FADE_SOURCE "control.audio.fade.source"
+#endif
+
 typedef struct _WpAudioStreamPrivate WpAudioStreamPrivate;
 struct _WpAudioStreamPrivate
 {