Log Message:
Updated png to 1.6.31.

Changes since the last public release (1.6.30):

  Guard the definition of _POSIX_SOURCE in pngpriv.h (AIX already defines it;
    bug report by Michael Felt).
  Revised pngpriv.h to work around failure to compile arm/filter_neon.S
    ("typedef" directive is unrecognized by the assembler). The problem
    was introduced in libpng-1.6.30beta01.
  Added "Requires: zlib" to libpng.pc.in (Pieter Neerincx).
  Added special case for FreeBSD in arm/filter_neon.S (Maya Rashish).
  Added instructions for disabling hardware optimizations in INSTALL.
  Added "--enable-hardware-optimizations" configuration flag to enable
    or disable all hardware optimizations with one flag.
  Updated CMakeLists.txt to add INTEL_SSE and MIPS_MSA platforms.
  Changed "int" to "png_size_t" in intel/filter_sse2.c to prevent
    possible integer overflow (Bug report by John Bowler).
  Quieted "declaration after statement" warnings in intel/filter_sse2.c.
  Added scripts/makefile-linux-opt, which has hardware optimizations enabled.
  Removed one of the GCC-7.1.0 'strict-overflow' warnings that result when
    integers appear on both sides of a compare.  Worked around the others by
    forcing the strict-overflow setting in the relevant functions to a level
    where they are not reported (John Bowler).
  Changed "FALL THROUGH" comments to "FALLTHROUGH" because \ 
GCC doesn't like
    the space.
  Worked around some C-style casts from (void*) because g++ 5.4.0 objects
    to them.
  Increased the buffer size for 'sprint' to pass the gcc 7.1.0 'sprint
    overflow' check that is on by default with -Wall -Wextra.
  Added eXIf chunk support.
  Added a minimal eXIf chunk (with Orientation and FocalLengthIn35mmFilm
    tags) to pngtest.png.