Log Message:
Updated png to 1.6.30.

Changes since the last public release (1.6.29):

  Added missing "$(CPPFLAGS)" to the compile line for c.pic.o in
    makefile.linux and makefile.solaris-x86 (Cosmin).
  Revised documentation of png_get_error_ptr() in the libpng manual.
  Silence clang -Wcomma and const drop warnings (Viktor Szakats).
  Update Sourceforge URLs in documentation (https instead of http).
  Document need to check for integer overflow when allocating a pixel
    buffer for multiple rows in contrib/gregbook, contrib/pngminus,
    example.c, and in the manual (suggested by Jaeseung Choi). This
    is similar to the bug reported against pngquant in CVE-2016-5735.
  Removed reference to the obsolete PNG_SAFE_LIMITS macro in the documentation.
  Check for integer overflow in contrib/visupng and contrib/tools/genpng.
  Do not double evaluate CMAKE_SYSTEM_PROCESSOR in CMakeLists.txt.
  Test CMAKE_HOST_WIN32 instead of WIN32 in CMakeLists.txt.
  Fix some URL in documentation.
  Avoid writing an empty IDAT when the last IDAT exactly fills the
    compression buffer (bug report by Brian Baird).  This bug was
    introduced in libpng-1.6.0.
  Update copyright year in pnglibconf.h, make ltmain.sh executable.
  Add a reference to the libpng.download site in README.