Log Message:
Update to 1.2.39. Use .xz instead of .gz. (20% smaller)

version 1.2.39beta01 [July 25, 2009]
  Added a prototype for png_64bit_product() in png.c

version 1.2.39beta02 [July 27, 2009]
  Avoid a possible NULL dereference in debug build, in png_set_text_2().
    (bug introduced in libpng-0.95, discovered by Evan Rouault)

version 1.2.39beta03 [July 29, 2009]
  Relocated new png_64_bit_product() prototype into png.h
  Expanded the information about prototypes in the libpng style section of
    the documentation.
  Rebuilt configure scripts with autoconf-2.64.

version 1.2.39beta04 [August 1, 2009]
  Replaced *.tar.lzma with *.txz in distribution.  Get the xz codec
    from <http://tukaani.org/xz>.

version 1.2.39beta05 [August 1, 2009]
  Reject attempt to write iCCP chunk with negative embedded profile length
    (JD Chen)

version 1.2.39c01 [August 6, 2009]
  No changes.

version 1.2.39 and 1.0.48 [August 13, 2009]
  No changes.