no links in PDFs with gs 9.56.1

[Werner LEMBERG]

Two days ago I've updated my openSUSE box, which now comes with gs
9.56.1 – previously, I had gs 9.54.0.  This new gs version uses a
brand-new PDF engine written in C, replacing the old one written in
PostScript.

Building the LilyPond documentation I wondered why the new output file
size of `notation.pdf` is only 75% of the old size.  After some checks
I found out that all links are gone.

So, a heads-up: gs 9.56.0 and 9.56.1 are buggy!  According to

  https://bugs.ghostscript.com/show_bug.cgi?id=705565

this has been fixed meanwhile, but I have no idea when 9.56.2 gets
released.

In case you are forced to use 9.56.1, add `-dNEWPDF=false` to the gs
command line options (in file `Documentation/GNUmakefile`) to make gs
use the old PDF engine, which produces good results.


    Werner