[png-mng-implement] libpng-1.5.17rc01 is available

[Glenn Randers-P. <gl...@gm...>]

libpng-1.5.17rc01 is available from
ftp://ftp.simplesystems.org/pub/png-group/src/libpng15
and from
http://libpng.sf.net

There are no changes from libpng-1.5.17beta01.

Glenn