XDG inaccessible as packaged by distros

[bryankaplan]

The enduser's ability to rely upon XDG base directories should not be hidden behind a compile-time flag (--enable-xdg), because package maintainers must choose compile flags to support a broad userbase — including users who've configured XDG base directories and those who have not.

Please either 1. make the flag a runtime flag, or 2. choose a default — either legacy dotfiles or XDG — and fall back to the other when the default does not exist.