[libpng16] Added #ifdef PNG_READ_SUPPORTED/endif around the

png_chunk_unknown_handling() function, and restored the unknown-chunk
handling code from libpng-1.6.6.

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pngrutil.c

@@ -2764,7 +2764,6 @@ png_handle_unknown(png_structrp png_ptr, png_inforp info_ptr,
 
    png_debug(1, "in png_handle_unknown");
 
-#ifdef PNG_READ_UNKNOWN_CHUNKS_SUPPORTED
    /* NOTE: this code is based on the code in libpng-1.4.12 except for fixing
     * the bug which meant that setting a non-default behavior for a specific
     * chunk would be ignored (the default was always used unless a user
@@ -2777,15 +2776,14 @@ png_handle_unknown(png_structrp png_ptr, png_inforp info_ptr,
     * function.
     */
 #  ifndef PNG_HANDLE_AS_UNKNOWN_SUPPORTED
-#     ifdef PNG_SET_UNKNOWN_CHUNKS_SUPPORTED
-         keep = png_chunk_unknown_handling(png_ptr, png_ptr->chunk_name);
-#     endif
+   keep = PNG_HANDLE_CHUNK_AS_DEFAULT;
 #  endif
 
    /* One of the following methods will read the chunk or skip it (at least one
     * of these is always defined because this is the only way to switch on
     * PNG_READ_UNKNOWN_CHUNKS_SUPPORTED)
     */
+#ifdef PNG_READ_UNKNOWN_CHUNKS_SUPPORTED
 #  ifdef PNG_READ_USER_CHUNKS_SUPPORTED
       /* The user callback takes precedence over the chunk keep value, but the
        * keep value is still required to validate a save of a critical chunk.
@@ -2893,7 +2891,7 @@ png_handle_unknown(png_structrp png_ptr, png_inforp info_ptr,
 
          png_crc_finish(png_ptr, length);
       }
-#  endif
+#  endif /* PNG_SAVE_UNKNOWN_CHUNKS_SUPPORTED */
 
 #  ifdef PNG_STORE_UNKNOWN_CHUNKS_SUPPORTED
       /* Now store the chunk in the chunk list if appropriate, and if the limits