snapcast: add pulseaudio support (#144674)

pkgs/applications/audio/snapcast/default.nix

···
 { stdenv, lib, fetchFromGitHub, cmake, pkg-config
 , alsa-lib, asio, avahi, boost17x, flac, libogg, libvorbis, soxr
+, pulseaudioSupport ? false, libpulseaudio
 , nixosTests }:
+
+assert pulseaudioSupport -> libpulseaudio != null;
 
 let
 
···
     boost17x
     alsa-lib asio avahi flac libogg libvorbis
     aixlog popl soxr
-  ];
+  ] ++ lib.optional pulseaudioSupport libpulseaudio;
 
   # Upstream systemd unit files are pretty awful, so we provide our own in a
   # NixOS module. It might make sense to get that upstreamed...

pkgs/top-level/all-packages.nix

···
 
   snallygaster = callPackage ../tools/security/snallygaster { };
 
-  snapcast = callPackage ../applications/audio/snapcast { };
+  snapcast = callPackage ../applications/audio/snapcast {
+    pulseaudioSupport = config.pulseaudio or stdenv.isLinux;
+  };
 
   snapdragon-profiler = callPackage ../tools/graphics/snapdragon-profiler { };