mpg321: fix build on darwin

pkgs/applications/audio/mpg321/default.nix

@@ -19,6 +19,11 @@ stdenv.mkDerivation rec {
       url = "https://sources.debian.org/data/main/m/mpg321/0.3.2-3/debian/patches/handle_illegal_bitrate_value.patch";
       sha256 = "15simp5fjvm9b024ryfh441rkh2d5bcrizqkzlrh07n9sm7fkw6x";
     })
+    # Apple defines semun already. Skip redefining it to fix build errors.
+    (fetchpatch {
+      url = "https://raw.githubusercontent.com/Homebrew/formula-patches/85fa66a9/mpg321/0.3.2.patch";
+      sha256 = "sha256-qFYpKpE9PZSzOJrnsQINZi6FvUVX0anRyOvlF5eOYqE=";
+    })
   ];
 
   hardeningDisable = [ "format" ];
@@ -37,6 +42,6 @@ stdenv.mkDerivation rec {
     description = "Command-line MP3 player";
     homepage = "http://mpg321.sourceforge.net/";
     license = licenses.gpl2;
-    platforms = platforms.gnu ++ platforms.linux;
+    platforms = platforms.unix;
   };
 }