exiv2: enable libpng, fix cmake files
exiv2: enable libpng, fix cmake files

Begasus/haikuports,haikuports/haikuports,Begasus/haikuports,haikuports/haikuports,Begasus/haikuports,haikuports/haikuports,haikuports/haikuports,moochris/haikuports,moochris/haikuports,Begasus/haikuports,moochris/haikuports,haikuports/haikuports,Begasus/haikuports