Make building cubeb optional

src/audio_core/CMakeLists.txt

@@ -4,8 +4,6 @@ add_library(audio_core STATIC
     audio_renderer.cpp
     audio_renderer.h
     buffer.h
-    cubeb_sink.cpp
-    cubeb_sink.h
     codec.cpp
     codec.h
     null_sink.h
@@ -15,6 +13,8 @@ add_library(audio_core STATIC
     sink_details.cpp
     sink_details.h
     sink_stream.h
+
+    $<$<BOOL:${ENABLE_CUBEB}>:cubeb_sink.cpp cubeb_sink.h>
 )
 
 create_target_directory_groups(audio_core)