fix: Fallback to readlink if macOS missing greadlink

The 'g' prefixed coreutils on macOS are installed through brew install coreutils. While most macOS systems are using homebrew, there are situations, like in conda-forge builds, where the use of homebrew isn't supported. As GNU readlink and homebrew's greadlink have the same API, fall back to using readlink if greadlink isn't on PATH.

This patch is needed as part of the conda-forge packaging (c.f. https://github.com/conda-forge/hepmc3-feedstock/pull/19).