Fantastic, Marius!  Some nits below:

>  ;;; Copyright © 2019 Marius Bakke <mbakke@fastmail.com>

2020 for Marius?

> +       ;; FIXME: libxml2 needs to built with icu support though it links to
> +       ;; libxml2 configure summary still states. Checking for compatible
> +       ;; system libxml2... no

Punctuation:

--8<---------------cut here---------------start------------->8---
... still states "Checking for compatible system libxml2... no".
--8<---------------cut here---------------end--------------->8---

> +       ;; Tests are disabled due to Could not find QtWebEngineProcess error
> +       ;; It's possible this can be fixed by setting QTWEBENGINEPROCESS_PATH
> +       ;; before running tests It's also possible some tests need a network

Punctuation:

--8<---------------cut here---------------start------------->8---
Test are disabled due to "Could not find QtWebEngineProcess" error.
It's possible this can be fixed by setting QTWEBENGINEPROCESS_PATH
before running tests.  It's also possible some tests need a network.
--8<---------------cut here---------------end--------------->8---

> +             (define (third-party? file)
> +               (if (string-contains file "third_party/")
> +                   #t
> +                   #f))

Maybe just

--8<---------------cut here---------------start------------->8---
(define (third-party? file)
  (string-contains file "third_party/"))
--8<---------------cut here---------------end--------------->8---

?

-- 
Pierre Neidhardt
https://ambrevar.xyz/