Maintainers of ports that list a libpng dependency

[Joshua Root]

Please check whether your ports still link against libpng after r115019
and other subsequent rev bumps. Dependencies on libpng were previously
added to a lot of ports that don't actually use it, but linked to it
because of a dependency that used it and libtool dependency_libs. A lot
of these probably can and should be removed now.

- Josh