If an application is intended to be installed by root on a system wide
basis then /usr/share is recommended to be used as value for
datadir and /etc/xdg is recommended to be
used as value for sysconfdir.
In case the /usr/share hierarchy is not writable it is recommended to
use /usr/local/share as value for datadir
instead.
If an application is intended to be installed by an unprivileged user
for exclusive use by that user only then
$XDG_DATA_HOME should be used as value
for datadir and
$XDG_CONFIG_HOME should be used as value
for sysconfdir.
If $XDG_DATA_HOME is not set, the default value of
$HOME/.local/share should be used for it.
If $XDG_CONFIG_HOME is not set, the default value of
$HOME/.config should be used for it.