https://github.com/Bots-United/metamod-hl1
This is a forked version of
metamod-am 1.21 (upstream appears unmaintained - pull requests ignored for ages):
- patched to build with latest version of Xcode under macOS
- added Xash3D engine support
Auto-built binaries can be downloaded from (for Windows, GNU/Linux and macOS):
https://dl.bintray.com/bots-united/metamod-hl1/