Log Message:
Update to 1.6.18:

Version 1.6.18beta01 [April 1, 2015]
  Removed PNG_SET_CHUNK_[CACHE|MALLOC]_LIMIT_SUPPORTED macros.  They
    have been combined with PNG_SET_USER_LIMITS_SUPPORTED (resolves
    bug report by Andrew Church).
  Fixed rgb_to_gray checks and added tRNS checks to pngvalid.c.  This
    fixes some arithmetic errors that caused some tests to fail on
    some 32-bit platforms (Bug reports by Peter Breitenlohner [i686]
    and Petr Gajdos [i586]).

Version 1.6.18beta02 [April 26, 2015]
  Suppressed some warnings from the Borland C++ 5.5.1/5.82 compiler
    (Bug report by Viktor Szaka'ts).

Version 1.6.18beta03 [May 6, 2015]
  Replaced "unexpected" with an integer (0xabadca11) in pngset.c
    where a long was expected, to avoid a compiler warning when PNG_DEBUG > 1.
  Added contrib/examples/simpleover.c, to demonstrate how to handle
    alpha compositing of multiple images, using the "simplified API"
    and an example PNG generation tool, contrib/examples/genpng.c
    (John Bowler).

Version 1.6.18beta04 [May 20, 2015]
  PNG_RELEASE_BUILD replaces tests where the code depended on the build base
    type and can be defined on the command line, allowing testing in beta
    builds (John Bowler).
  Avoid Coverity issue 80858 (REVERSE NULL) in pngtest.c PNG_DEBUG builds.
  Avoid a harmless potential integer overflow in png_XYZ_from_xy() (Bug
    report from Christopher Ferris).

Version 1.6.18beta05 [May 31, 2015]
  Backport filter selection code from libpng-1.7.0beta51, to combine
    sub_row, up_row, avg_row, and paeth_row into try_row and tst_row.
  Changed png_voidcast(), etc., to voidcast(), etc., in contrib/tools/pngfix.c
    to avoid confusion with the libpng private macros.
  Fixed old cut&paste bug in the weighted filter selection code in
    pngwutil.c, introduced in libpng-0.95, March 1997.

Version 1.6.18beta06 [June 1, 2015]
  Removed WRITE_WEIGHTED_FILTERED code, to save a few kbytes of the
    compiled library size. It never worked properly and as far as we can
    tell, no one uses it. The png_set_filter_heuristics() and
    png_set_filter_heuristics_fixed() APIs are retained but deprecated
    and do nothing.

Version 1.6.18beta07 [June 6, 2015]
  Removed non-working progressive reader 'skip' function. This
    function has apparently never been used. It was implemented
    to support back-door modification of png_struct in libpng-1.4.x
    but (because it does nothing and cannot do anything) was apparently
    never tested (John Bowler).
  Fixed cexcept.h in which GCC 5 now reports that one of the auto
    variables in the Try macro needs to be volatile to prevent value
    being lost over the setjmp (John Bowler).
  Fixed NO_WRITE_FILTER and -Wconversion build breaks (John Bowler).
  Fix g++ build breaks (John Bowler).
  Quieted some Coverity issues in pngfix.c, png-fix-itxt.c, pngvalid.c,
    pngstest.c, and pngimage.c. Most seem harmless, but png-fix-itxt
    would only work with iTXt chunks with length 255 or less.
  Added #ifdef's to contrib/examples programs so people don't try
    to compile them without the minimum required support enabled
    (suggested by Flavio Medeiros).

Version 1.6.18beta08 [June 30, 2015]
  Eliminated the final two Coverity defects (insecure temporary file
    handling in contrib/libtests/pngstest.c; possible overflow of
    unsigned char in contrib/tools/png-fix-itxt.c). To use the "secure"
    file handling, define PNG_USE_MKSTEMP, otherwise "tmpfile()" will
    be used.
  Removed some unused WEIGHTED_FILTER macros from png.h and pngstruct.h

Version 1.6.18beta09 [July 5, 2015]
  Removed some useless typecasts from contrib/tools/png-fix-itxt.c
  Fixed a new signed-unsigned comparison in pngrtran.c (Max Stepin).
  Replaced arbitrary use of 'extern' with #define PNG_LINKAGE_*.  To
    preserve API compatibility, the new defines all default to "extern"
    (requested by Jan Nijtmans).

Version 1.6.18rc01 [July 9, 2015]
  Belatedly added Mans Rullgard and James Yu to the list of Contributing
    Authors.

Version 1.6.18rc02 [July 12, 2015]
  Restored unused FILTER_HEURISTIC macros removed at libpng-1.6.18beta08
    to png.h to avoid compatibility warnings.

Version 1.6.18rc03 [July 15, 2015]
  Minor changes to the man page

Version 1.6.18 [July 23, 2015]
  No changes.