You need Xcode/Xcode Command line tools installed. You can get a list of direct download links hosted by Apple from xcodereleases.com. You also need either MacPorts (recommended) or Homebrew on your Mac.
1) Clone recursively from Github:
git clone --recursive https://github.com/alex-free/source-engine-mac-app
2) Change into the source directory:
cd source-engine-mac-app
3) Execute ./build to generate a portable zip release.
If you want to clean the built release, execute ./build clean.