Log Message:
Update png to 1.2.12, for security fixes. ok schmonz@, jlam@.

version 1.2.11beta1 [May 31, 2006]
  scripts/libpng.pc.in contained "configure" style version info and would
    not work with makefiles.
  The shared-library makefiles were linking to libpng.so.0 instead of
    libpng.so.3 compatibility as the library.

version 1.2.11beta2 [June 2, 2006]
  Increased sprintf buffer from 50 to 52 chars in pngrutil.c to avoid
    buffer overflow.
  Fixed bug in example.c (png_set_palette_rgb -> png_set_palette_to_rgb)

version 1.2.11beta3 [June 5, 2006]
  Prepended "#! /bin/sh" to ltmail.sh and contrib/pngminus/*.sh (Cosmin).
  Removed the accidental leftover Makefile.in~ (Cosmin).
  Avoided potential buffer overflow and optimized buffer in
    png_write_sCAL(), png_write_sCAL_s() (Cosmin).
  Removed the include directories and libraries from CFLAGS and LDFLAGS
    in scripts/makefile.gcc (Nelson A. de Oliveira, Cosmin).

version 1.2.11beta4 [June 6, 2006]
  Allow zero-length IDAT chunks after the entire zlib datastream, but not
    after another intervening chunk type.