otherwise custom link-exe templates do not work

if a user has XDG_* paths setup on mac, it's more than likely those were
explicitly defined and therefore the preferred mechanism for organizing
such fs data. the ~/Library paths are sensible default alternatives,
but providing a way to override them (via the environment rather than
command-line arguments) is not only convenient but also consistent with
linux/fbsd path behavior

~/Preferences is not a thing

- Add a getenv() abstraction that is more versatile than the std:: one