Fix build with png-1.5.

--- src/pilot-read-veo.c.orig	Thu Jun  4 07:26:19 2009
+++ src/pilot-read-veo.c	Mon Jul  4 15:50:37 2011
@@ -41,10 +41,6 @@
 
 #ifdef HAVE_PNG
 # include "png.h"
-# if (PNG_LIBPNG_VER < 10201)
-#  define png_voidp_NULL (png_voidp)NULL
-#  define png_error_ptr_NULL (png_error_ptr)NULL
-# endif
 #endif
 
 #define pi_mktag(c1,c2,c3,c4) (((c1)<<24)|((c2)<<16)|((c3)<<8)|(c4))
@@ -856,8 +852,8 @@ void write_png (FILE * f, struct Veo *v, long flags)
    png_infop info_ptr;
 
    png_ptr = png_create_write_struct
-	 (PNG_LIBPNG_VER_STRING, png_voidp_NULL,
-	  png_error_ptr_NULL, png_error_ptr_NULL);
+	 (PNG_LIBPNG_VER_STRING, NULL,
+	  NULL, NULL);
 
    if (!png_ptr)
 	 return;
