Log Message:
Update to 1.2.18:

version 1.2.17beta1 [March 6, 2007]
  Revised scripts/CMakeLists.txt to install both shared and static libraries.
  Deleted a redundant line from pngset.c.

version 1.2.17beta2 [April 26, 2007]
  Relocated misplaced test for png_ptr == NULL in pngpread.c
  Change "==" to "&" for testing PNG_RGB_TO_GRAY_ERR \ 
& PNG_RGB_TO_GRAY_WARN
    flags.
  Changed remaining instances of PNG_ASSEMBLER_* to PNG_MMX_*
  Added pngerror() when write_IHDR fails in deflateInit2().
  Added "const" to some array declarations.
  Mention examples of libpng usage in the libpng*.txt and libpng.3 documents.

version 1.2.17rc1 [May 4, 2007]
  No changes.

version 1.2.17rc2 [May 8, 2007]
  Moved several PNG_HAVE_* macros out of PNG_INTERNAL because applications
    calling set_unknown_chunk_location() need them.
  Changed transformation flag from PNG_EXPAND_tRNS to PNG_EXPAND in
    png_set_expand_gray_1_2_4_to_8().
  Added png_ptr->unknown_chunk to hold working unknown chunk data, so it
    can be free'ed in case of error.  Revised unknown chunk handling in
    pngrutil.c and pngpread.c to use this structure.

version 1.2.17rc3 [May 8, 2007]
  Revised symbol-handling in configure script.

version 1.2.17rc4 [May 10, 2007]
  Revised unknown chunk handling to avoid storing unknown critical chunks.

version 1.2.17 [May 15, 2007]
  Added "png_ptr->num_trans=0" before error return in png_handle_tRNS,
    to eliminate a vulnerability (CVE-2007-2554, CERT VU#684664)

version 1.2.18 [May 15, 2007]
  Reverted the libpng-1.2.17rc3 change to symbol-handling in configure script