Fix build with png-1.5.

--- my_png.cpp.orig	Mon Feb  1 13:31:34 2010
+++ my_png.cpp	Thu Oct 27 14:50:22 2016
@@ -53,7 +53,7 @@ void my_png_save_img(image_info* img, char* filename)
         return;
     }
 
-    if (setjmp(png_ptr->jmpbuf)) {
+    if (setjmp(png_jmpbuf(png_ptr))) {
         fprintf(stderr, "Internal error in libpng\n");
         if (png_pal)
             delete[] png_pal;