Similar to XDG_DATA_DIRS defaulting to KDE_INSTALL_FULL_DATAROOTDIR, and
some sensible constants, also make XDG_CONFIG_DIRS default to KDE_INSTALL_FULL_CONFDIR
and some sensible constants.
Please make sure the first line of the commit log says CONFIG and not DATA, too (the phab title still says DATA).
Wait, where does /usr/local/etc/xdg come from? https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html says the default is only /etc/xdg.
On FreeBSD (other BSD's, too), /etc is for the base system only, and all user-add-ons go in *$LOCALBASE*, usually /usr/local .. so XDG stuff, which is a user-add-on, goes into /usr/local/etc. Even our /etc/os-release is in /usr/local/etc/os-release (and Frameworks are suitably patched for that).
Some BSD derivatives are less dogmatic about /etc, but not us.