Log Message:
Update to 1.6.13:

Version 1.6.13beta01 [July 4, 2014]
  Quieted -Wsign-compare and -Wclobber compiler warnings in
    contrib/pngminus/*.c
  Added "(void) png_ptr;" where needed in contrib/gregbook to quiet
    compiler complaints about unused pointers.
  Split a long output string in contrib/gregbook/rpng2-x.c.
  Added "PNG_SET_OPTION" requirement for sRGB chunk support to \ 
pnglibconf.dfa,
    Needed for write-only support (John Bowler).
  Changed "if defined(__ARM_NEON__)" to
    "if (defined(__ARM_NEON__) || defined(__ARM_NEON))" (James Wu).
  Fixed clang no-warning builds: png_digit was defined but never used.

Version 1.6.13beta02 [July 21, 2014]
  Fixed an incorrect separator ("/" should be "\") in \ 
scripts/makefile.vcwin32
    (bug report from Wolfgang S. Kechel).  Bug was introduced in libpng-1.6.11.
    Also fixed makefile.bc32, makefile.bor, makefile.msc, makefile.intel, and
    makefile.tc3 similarly.

Version 1.6.13beta03 [August 3, 2014]
  Removed scripts/makefile.elf. It has not worked since libpng-1.5.0beta14
    due to elimination of the PNG_FUNCTION_EXPORT and PNG_DATA_EXPORT
    definitions from pngconf.h.
  Ensure that CMakeLists.txt makes the target "lib" directory before making
    symbolic link into it (SourceForge bug report #226 by Rolf Timmermans).

Version 1.6.13beta04 [August 8, 2014]
  Added opinion that the ECCN (Export Control Classification Number) for
    libpng is EAR99 to the README file.
  Eliminated use of "$<" in makefile explicit rules, when copying
    $PNGLIBCONF_H_PREBUILT.  This does not work on some versions of make;
    bug introduced in libpng version 1.6.11.

Version 1.6.13rc01 [August 14, 2014]
  Made "ccopts" agree with "CFLAGS" in scripts/makefile.hp* \ 
and makefile.*sunu

Version 1.6.13 [August 21, 2014]
  No changes.