tcl: Separate man output and use symlinks

This reduces tcl's total size from 25.0 MiB to 8.6 MiB. Admittedly
this is also because putting the manpages in the right place causes
all man3 pages to be deleted by the multiple outputs setup hook. Not
sure if that's desirable behaviour...

pkgs/development/interpreters/tcl/generic.nix

@@ -10,10 +10,18 @@ stdenv.mkDerivation rec {
 
   inherit src;
 
+  outputs = [ "out" "man" ];
+
+  setOutputFlags = false;
+
   preConfigure = ''
+    # Note: using $out instead of $man to prevent a runtime dependency on $man.
+    configureFlagsArray+=(--mandir=$out/share/man --enable-man-symlinks)
     cd unix
   '';
 
+  enableParallelBuilding = true;
+
   postInstall = ''
     make install-private-headers
     ln -s $out/bin/tclsh${release} $out/bin/tclsh