work with newer png library now

Fixed a compiler issue when using newer png

libraries/bin/sh ../libtool --tag=CXX   --mode=compile g++ -DHAVE_CONFIG_H -I. -I..  -I./../include -DLIBPLOT -DLIBPLOTTER    -O2 -MT x_retrieve.lo -MD -MP -MF .deps/x_retrieve.Tpo -c -o x_retrieve.lo x_retrieve.cc
z_write.cc: In function 'void _our_error_fn_stream(png_struct*, const char*)':
z_write.cc:520:19: error: invalid use of incomplete type 'png_struct {aka struct png_struct_def}'
   longjmp (png_ptr->jmpbuf, 1);
                   ^
In file included from z_write.cc:43:0:
/usr/local/include/png.h:475:16: note: forward declaration of 'png_struct {aka struct png_struct_def}'
 typedef struct png_struct_def png_struct;
                ^