bug#60201: [PATCH] gnu: Add libswell

[Ricardo Wurmus]

Thank you for your patience.
I took the liberty of making a few changes:

- minor indentation / formatting changes
- pulled out the “with-directory-excursion” to simplify the snippet
- used “with-directory-excursion” where possible to avoid stateful
  behavior across build phases.
- removed the “mkdir-p” which would have only created empty directories
  in $out/include, and simplified installation of all headers.
- added a copyright line for you

It’s all pushed to the “master” branch with commit
a8f15711508abab85cba3174f68a4343a15fb116.

Thanks!

-- 
Ricardo