tk: Separate man output

This reduces the size of the main output by half a megabyte.

pkgs/development/libraries/tk/generic.nix

@@ -5,14 +5,19 @@ stdenv.mkDerivation {
 
   inherit src patches;
 
-  postInstall = ''
-    ln -s $out/bin/wish* $out/bin/wish
-  '';
+  outputs = [ "out" "man" ];
+
+  setOutputFlags = false;
 
   preConfigure = ''
+    configureFlagsArray+=(--mandir=$man/share/man --enable-man-symlinks)
     cd unix
   '';
 
+  postInstall = ''
+    ln -s $out/bin/wish* $out/bin/wish
+  '';
+
   configureFlags = [
     "--with-tcl=${tcl}/lib"
   ];