[libpng15] #ifdef out png_progressive_combine_row() when interlacing is

not supported.
diff --git a/example.c b/example.c
index c0d27f4..fc91f4a 100644
--- a/example.c
+++ b/example.c
@@ -2,7 +2,7 @@
 #if 0 /* in case someone actually tries to compile this */
 
 /* example.c - an example of using libpng
- * Last changed in libpng 1.5.4 [July 7, 2011]
+ * Last changed in libpng 1.5.6 [(PENDING RELEASE)]
  * This file has been placed in the public domain by the authors.
  * Maintained 1998-2011 Glenn Randers-Pehrson
  * Maintained 1996, 1997 Andreas Dilger)
@@ -341,11 +341,16 @@
    /* Add filler (or alpha) byte (before/after each RGB triplet) */
    png_set_filler(png_ptr, 0xff, PNG_FILLER_AFTER);
 
+#ifdef PNG_READ_INTERLACING_SUPPORTED
    /* Turn on interlace handling.  REQUIRED if you are not using
     * png_read_image().  To see how to handle interlacing passes,
     * see the png_read_row() method below:
     */
    number_passes = png_set_interlace_handling(png_ptr);
+#else
+   number_passes = 1;
+#endif /* PNG_READ_INTERLACING_SUPPORTED */
+
 
    /* Optional call to gamma correct and add the background to the palette
     * and update info structure.  REQUIRED if you are expecting libpng to
@@ -527,6 +532,7 @@
     */
    png_bytep old_row = ((png_bytep *)our_data)[row_num];
 
+#ifdef PNG_READ_INTERLACING_SUPPORTED
    /* If both rows are allocated then copy the new row
     * data to the corresponding row data.
     */
@@ -555,6 +561,7 @@
     * to pass the current row as new_row, and the function will combine
     * the old row and the new row.
     */
+#endif /* PNG_READ_INTERLACING_SUPPORTED */
 }
 
 end_callback(png_structp png_ptr, png_infop info)