Use the `png_jmpbuf' accessor, as recommended since libpng 1.4.0:
http://www.libpng.org/pub/png/src/libpng-1.2.x-to-1.4.x-summary.txt .

--- plotutils-2.6/libplot/z_write.c	2013-07-12 17:19:12.000000000 +0200
+++ plotutils-2.6/libplot/z_write.c	2013-07-12 17:19:07.000000000 +0200
@@ -164,7 +164,7 @@ _pl_z_maybe_output_image (S___(Plotter *
     }
 
   /* cleanup after libpng errors (error handler does a longjmp) */
-  if (setjmp (png_ptr->jmpbuf))
+  if (setjmp (png_jmpbuf (png_ptr)))
     {
       png_destroy_write_struct (&png_ptr, (png_info **)NULL);
       return -1;
@@ -444,7 +444,7 @@ _our_error_fn_stdio (png_struct *png_ptr
 #endif
     }
 
-  longjmp (png_ptr->jmpbuf, 1);
+  longjmp (png_jmpbuf (png_ptr), 1);
 }
 
 static void 
elected'>koszko</option>
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<tr><td><span title='2020-07-10 00:11:00 +0200'>2020-07-10</span></td><td><a href='/guix/commit/etc/substitutes?id=d283bb960f927dd5f7bb8b96bc697221e4e8ad39'>maint: Remove traces of "berlin.guixsd.org".</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>