Weston should fallback if XDG_RUNTIME_DIR is not set as per the XDG specification

[Fred .]

http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html#variables

"If $XDG_RUNTIME_DIR is not set applications should fall back to a
replacement directory with similar capabilities and print a warning
message."

Weston produces the following error message when XDG_RUNTIME_DIR is not set.
[21:43:53.365] fatal: environment variable XDG_RUNTIME_DIR is not set.