[Apologies for the premature send on the previous.]

Unfortunately, that's not the correct fix.  The code that's causing the
error shouldn't be compiled at all on an NS build; it's in a large block
that's conditioned on HAVE_PNG, which the NS build expects to be false, as
that port supplies its on version of png_load().

The current incorrect configuration behavior appears to have been caused by:

revno: 117059
fixes bug: http://debbugs.gnu.org/17339
committer: Paul Eggert <eggert@cs.ucla.edu>
branch nick: trunk
timestamp: Sun 2014-05-04 14:28:08 -0700
message:
  Consult libpng-config more consistently.

  This is mainly for simplicity, but it should also avoid
  some future problems like the ones we recently had with NetBSD.
  * configure.ac (LIBPNG): Configure after LIBZ.  Use libpng-config
  for cflags, too.  Append -lz if we're not already doing that with
  LIBZ.  Do not bother appending -lm, since we always append that.
  Coalesce some duplicate code.
  * src/Makefile.in (PNG_CFLAGS): New var.
  (ALL_CFLAGS): Use it.
  * src/image.c [HAVE_PNG]: Don't worry about <libpng/png.h>, as
  CFLAGS now handles this.