mpd: fix build on darwin

pkgs/servers/mpd/clientlib.nix

@@ -20,7 +20,7 @@ stdenv.mkDerivation rec {
     description = "Client library for MPD (music player daemon)";
     homepage = http://mpd.wikia.com/wiki/Music_Player_Daemon_Wiki;
     license = "GPLv2";
-    platforms = with stdenv.lib.platforms; linux;
+    platforms = with stdenv.lib.platforms; unix;
     maintainers = [ stdenv.lib.maintainers.mornfall ];
   };
 }

pkgs/servers/mpd/default.nix

@@ -80,6 +80,7 @@ in stdenv.mkDerivation rec {
       (mkFlag mmsSupport "mms")
       (mkFlag mpg123Support "mpg123")
       (mkFlag aacSupport "aac")
+      (mkFlag stdenv.isDarwin "osx")
       "--enable-debugging" ]
     ++ opt stdenv.isLinux
       "--with-systemdsystemunitdir=$(out)/etc/systemd/system";