$NetBSD$

pkg-config will take care with version id like:
-lpng15 
And simply -lpng is now harmful

--- configure.in.orig	2011-08-10 10:32:39.000000000 +0900
+++ configure.in	2011-08-10 10:46:11.000000000 +0900
@@ -2458,7 +2458,7 @@
 
   if test "${HAVE_PNG}" = "yes"; then
     AC_DEFINE(HAVE_PNG, 1, [Define to 1 if you have the png library (-lpng).])
-    LIBPNG="-lpng -lz -lm"
+    LIBPNG="-lz -lm"
   fi
 fi
 AC_SUBST(LIBPNG)