First, add the zxe repository and update the package index:

curl -L https://zxe.io/scripts/add-zxe-apt-repo.sh | sudo sh
sudo apt update

Next, install the library package:

sudo apt install libz80

In case you need to build software that requires the Z80 library, install the development package too:

sudo apt install libz80-dev

First, add and sync the zxe overlay:

eselect repository add zxe git https://github.com/redcode/zxe-gentoo-overlay.git
emaint sync --repo zxe

Then install the library:

emerge emulation-libs/z80

brew install redcode/zxe/z80

Pre-built binaries for Windows are available on the download page.