ploticus: fixup a manual-page symlink

With new man compression this caused a build error.

pkgs/tools/graphics/ploticus/default.nix

@@ -15,6 +15,12 @@ stdenv.mkDerivation {
 
   patches = [ ./ploticus-install.patch ];
 
+  # Make the symlink relative instead of absolute.
+  # Otherwise it breaks when auto-moved to $out/share.
+  preFixup = ''
+    ln -sf pl.1 "$out"/man/man1/ploticus.1
+  '';
+
   meta = with stdenv.lib; {
     description = "A non-interactive software package for producing plots and charts";
     longDescription = ''Ploticus is a free, GPL'd, non-interactive