dirdiff: add darwin support

pkgs/tools/text/dirdiff/default.nix

···
     done
   '';
 
+  env = {
+    NIX_CFLAGS_COMPILE = "-DUSE_INTERP_RESULT";
+    NIX_LDFLAGS = "-ltcl";
+  };
+
   # If we don't create the directories ourselves, then 'make install' creates
   # files named 'bin' and 'lib'.
   preInstall = ''
···
     homepage = "https://www.samba.org/ftp/paulus/";
     license = licenses.gpl2Plus;
     maintainers = with maintainers; [ khumba ];
-    platforms = platforms.linux;
+    platforms = platforms.unix;
   };
 }

pkgs/top-level/all-packages.nix

···
 
   dsp = callPackage ../tools/audio/dsp { };
 
-  dirdiff = callPackage ../tools/text/dirdiff {
-    tcl = tcl-8_5;
-    tk = tk-8_5;
-  };
+  dirdiff = callPackage ../tools/text/dirdiff { };
 
   dwdiff = callPackage ../applications/misc/dwdiff { };