Update libpng to 1.6.10.

Fix --enable-localfoo options.


git-svn-id: http://svn.msweet.org/htmldoc@1721 d512abc9-c3ee-0310-a016-ca57acef63f3
Add changelog for configure script fix.


git-svn-id: http://svn.msweet.org/htmldoc@1722 d512abc9-c3ee-0310-a016-ca57acef63f3