Support build with libpng version 1.5.x.

--- zvbi-0.2.33-orig/src/exp-gfx.c	2012-02-18 03:20:39.668146729 +0100
+++ zvbi-0.2.33/src/exp-gfx.c	2012-02-18 03:08:29.578283492 +0100
@@ -1637,6 +1637,19 @@ VBI_AUTOREG_EXPORT_MODULE(vbi_export_cla
 #include "png.h"
 #include "setjmp.h"
 
+/* starting from libpng version 1.5 it is not possible
+ * to access inside the PNG struct directly
+ */
+#ifdef PNG_SETJMP
+#undef PNG_SETJMP
+#endif
+
+#if (defined(PNG_LIBPNG_VER) && PNG_LIBPNG_VER >= 10500)
+#define PNG_SETJMP(ptr) setjmp(png_jmpbuf(ptr))
+#else
+#define PNG_SETJMP(ptr) setjmp(ptr->jmpbuf)
+#endif
+
 static void
 write_data			(png_structp		png_ptr,
 				 png_bytep		data,
@@ -1672,11 +1685,11 @@ write_png			(gfx_instance *		gfx,
 	char title[80];
 	unsigned int i;
 
-	if (setjmp (png_ptr->jmpbuf))
+	if (PNG_SETJMP(png_ptr))
 		return FALSE;
 
 	png_set_write_fn (png_ptr,
-			  (voidp) gfx,
+			  (png_voidp) gfx,
 			  write_data,
 			  flush_data);