The %S formatting of file names ensures the path is quoted, allowing for
paths with spaces. Before constructing an include based on the file name
though, we now first check that the buffer in fact has a file name.
`org-glossary--include-once` calls `org-export--prepare-file-contents`,
which is defined in `ox`, but `ox` is not loaded.
This happens because `org-export--prepare-file-contents` (defined in
`ox`) is called in `org-glossary--include-once`, which is called if
org-glossary-mode is given an external source to look up terms in. E.g.
(setq org-glossary-global-terms (list "~/path/to/glossary.org"))