Z88dk is a rapidly evolving project. Even since the last release in January there have been more than a thousand commits to the repository and this includes improvements applied to the compilers and libraries.
It's best to try to keep relatively up-to-date and you can do that by either grabbing the nightly build or, if you're comfortable with building the libraries and binaries yourself, go through github and retain the ability to contribute. Install instructions are here. You can update a nightly build install for windows or mac os x by deleting the z88dk tree and unzipping a current build in the same location. This way you don't have to change any environment variables.