libpng 1.6.39-1

[Ken Brown via Cygwin-announce]

The following packages have been uploaded to the Cygwin distribution:
* libpng16-1.6.39-1
* libpng16-devel-1.6.39-1
* libpng-devel-1.6.39-1
* libpng-tools-1.6.39-1

libpng is the official reference library for the Portable Network 
Graphics (PNG) image format.

This is an update to the latest upstream release.

Ken