requested to merge arichardson/extra-cmake-modules:add-usr-local-etc-xdg into master
I can see that this directory is non-empty on at least FreeBSD, so we should add it in the same way as we add /usr/local/share to XDG_DATA_DIRS.
While the spec says
If $XDG_CONFIG_DIRS is either not set or empty, a value equal to /etc/xdg should be used that seems inconsistent with adding
/usr/local/share to the XDG_DATA_DIRS default.