The library contains its own inflate implementation (inflate = deflate decompression). The implementation does not depend on Allegro nor AllegroPNG so it can be used standalone. It makes no sense to do so, because original zlib implementation is much faster, has also compression and it's under same licence. Or, if you are looking for public domain zlib, there is Crypto++. :-) However if you are enough crazy, you can find the file in the Subversion repository or in the download area.