Update to 1.6.16; a CVE was requested for a heap overflow.

Changes since the last public release (1.6.15):
  Added ".align 2" to arm/filter_neon.S to support old GAS assemblers that
    don't do alignment correctly.
  Revised Makefile.am and scripts/*.dfn to work with MinGW/MSYS;
    renamed scripts/*.dfn to scripts/*.c (Bob Friesenhahn and John Bowler).
  Quiet a "comparison always true" warning in pngstest.c (John Bowler).
  Restored a test on width that was removed from png.c at libpng-1.6.9
    (Bug report by Alex Eubanks).
  Fixed an overflow in png_combine_row with very wide interlaced images.
Updated graphics/png to 1.6.16