[libpng17] Reverted the change to unknown handling #defines; the change

breaks 'NOREAD' builds.

pngrutil.c

@@ -2746,6 +2746,7 @@ 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
@@ -2758,14 +2759,15 @@ png_handle_unknown(png_structrp png_ptr, png_inforp info_ptr,
     * function.
     */
 #  ifndef PNG_HANDLE_AS_UNKNOWN_SUPPORTED
-   keep = png_chunk_unknown_handling(png_ptr, png_ptr->chunk_name);
+#     ifdef PNG_SET_UNKNOWN_CHUNKS_SUPPORTED
+         keep = png_chunk_unknown_handling(png_ptr, png_ptr->chunk_name);
+#     endif
 #  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.
@@ -2851,7 +2853,7 @@ png_handle_unknown(png_structrp png_ptr, png_inforp info_ptr,
 
          png_crc_finish(png_ptr, length);
       }
-#  endif /* PNG_SAVE_UNKNOWN_CHUNKS_SUPPORTED */
+#  endif
 
 #  ifdef PNG_STORE_UNKNOWN_CHUNKS_SUPPORTED
       /* Now store the chunk in the chunk list if appropriate, and if the limits