Comment 5 for bug 414594

It occurred to me recently that we could fix this more-fully by setting these vars in the resulting PREFIX/bin/exaile script. we already do a similar action to this to set the path to the main exaile.py script. Logic for the additional code would be something like

if xdg_var is set, xdg_var=xdg_var:custom_location
else, xdg_var=xdg_defaults:custom_location

for both XDG_CONFIG_DIRS and XDG_DATA_DIRS