prefix.sh: Add /usr/local/etc/xdg to the default XDG_CONFIG_DIRS
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.