Description of problem:
rpmlint does not accept zlib/libpng as a license tag but zlib. Zlib/libpng seems
to be the correct name of this license (see
http://www.opensource.org/licenses/zlib-license.php) so in my oppinion rpmlint
should accept zlib/libpng instead of zlib.
Fixed upstream, will be in the next rpmlint package revision.
Fixed in 0.78-1.