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);
 }
 
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<tr><td><span title='2022-06-15 00:25:21 +0200'>2022-06-15</span></td><td><a href='/guix/commit/gnu/services/authentication.scm?id=8cb1a49a3998c39f315a4199b7d4a121a6d66449'>services: configuration: Use *unspecified* instead of 'disabled.</a><span class='msg-avail'>...</span></td><td>Attila Lendvai</td></tr>
<tr><td><span title='2021-11-30 01:08:55 +0100'>2021-11-30</span></td><td><a href='/guix/commit/gnu/services/authentication.scm?id=892f1b7273d57b25940700877a02618fe826cc08'>services: Accept &lt;inferior-package&gt;s in lieu of &lt;package&gt;s.</a><span class='msg-avail'>...</span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2021-03-10 18:01:47 +0100'>2021-03-10</span></td><td><a href='/guix/commit/gnu/services/authentication.scm?id=520bac7ed00a949a0391ad680de65a1498105c2b'>services: Prevent following symlinks during activation.</a><span class='msg-avail'>...</span></td><td>Maxime Devos</td></tr>