fix png compile error with mapnik head (post https://github.com/mapnik/mapnik/commit/cd7ad3e15eed5b9b582c2b92175a68b77c1f5547)

gistfile1.txt

diff --git a/src/png_reader.cpp b/src/png_reader.cpp
index 8b993c7..15a8fbd 100644
--- a/src/png_reader.cpp
+++ b/src/png_reader.cpp
@@ -119,7 +119,8 @@ void png_reader::init()
         throw image_reader_exception("failed to create info_ptr");
     }
 
-    if (setjmp(png_jmpbuf(png_ptr)))
+    jmp_buf* jmp_context = (jmp_buf*) png_get_error_ptr(png_ptr);
+    if (jmp_context)
     {
         png_destroy_read_struct(&png_ptr,0,0);
         fclose(fp);
@@ -175,7 +176,8 @@ void png_reader::read(unsigned x0, unsigned y0,image_data_32& image)
         throw image_reader_exception("failed to create info_ptr");
     }
 
-    if (setjmp(png_jmpbuf(png_ptr)))
+    jmp_buf* jmp_context = (jmp_buf*) png_get_error_ptr(png_ptr);
+    if (jmp_context)
     {
         png_destroy_read_struct(&png_ptr,0,0);
         fclose(fp);