Changes 1.5.3beta05:
* Added the "_POSIX_SOURCE" feature test macro to ensure libpng sees the
  correct API. _POSIX_SOURCE is defined in pngpriv.h, pngtest.c and
  pngvalid.c to ensure that POSIX conformant systems disable non-POSIX APIs.
* Removed png_snprintf and added formatted warning messages.  This change adds
  internal APIs to allow png_warning messages to have parameters without
  requiring the host OS to implement snprintf.  As a side effect the
  dependency of the tIME-supporting RFC1132 code on stdio is removed and
  PNG_NO_WARNINGS does actually work now.
* Added PNG_WRITE_OPTIMIZE_CMF_SUPPORTED macro to make the zlib "CMF" byte
  optimization configureable.
* IDAT compression failed if preceded by a compressed text chunk (bug
  introduced in libpng-1.5.3beta01-02).  This was because the attempt to
  reset the zlib stream in png_write_IDAT happened after the first IDAT
  chunk had been deflated - much too late.  In this change internal
  functions were added to claim/release the z_stream and, hopefully, make
  the code more robust.  Also deflateEnd checking is added - previously
  libpng would ignore an error at the end of the stream.

(adam)