nixos/mpd: allow storage plugins in musicDirectory

nixos/modules/services/audio/mpd.nix

···
       };
 
       musicDirectory = mkOption {
-        type = types.path;
+        type = with types; either path (strMatching "(http|https|nfs|smb)://.+");
         default = "${cfg.dataDir}/music";
         defaultText = ''''${dataDir}/music'';
         description = ''
-          The directory where mpd reads music from.
+          The directory or NFS/SMB network share where mpd reads music from.
         '';
       };
 

pkgs/servers/mpd/default.nix

···
     ++ opt clientSupport mpd_clientlib
     ++ opt opusSupport libopus
     ++ opt soundcloudSupport yajl
-    ++ opt nfsSupport libnfs
-    ++ opt smbSupport smbclient;
+    ++ opt (!stdenv.isDarwin && nfsSupport) libnfs
+    ++ opt (!stdenv.isDarwin && smbSupport) smbclient;
 
   nativeBuildInputs = [ autoreconfHook pkgconfig ];
 
···
       (mkFlag clientSupport "libmpdclient")
       (mkFlag opusSupport "opus")
       (mkFlag soundcloudSupport "soundcloud")
-      (mkFlag nfsSupport "libnfs")
-      (mkFlag smbSupport "smbclient")
+      (mkFlag (!stdenv.isDarwin && nfsSupport) "libnfs")
+      (mkFlag (!stdenv.isDarwin && smbSupport) "smbclient")
       "--enable-debug"
       "--with-zeroconf=avahi"
     ]