The "asdf-build-system/source" workaround seems to have a limitation
though: it does not include its input recursively.

For instance, if I try to build Next that depends on cl-dbus, it will
complain about missing dependencies for cl-dbus.
Adding

--8<---------------cut here---------------start------------->8---
       ,@(package-inputs cl-dbus)
--8<---------------cut here---------------end--------------->8---

to the list of dependencies fixes it, but that's not pretty.

I believe that this is a general issue with asdf-build-system/source
(could be wrong).
If I look at the environment-variables file when building Next, it has

--8<---------------cut here---------------start------------->8---
export XDG_DATA_DIRS=\
"/gnu/store/2fkvzws2vqhd7zq265n2a3rjkdmc0i6v-sbcl-1.5.1/share\
:/gnu/store/qc74s7g1z3pm11d0kfka9pkc5af11pa6-sbcl-prove-asdf-1.0.0-1.4f9122b/share\
:/gnu/store/aglych5kld05hip5j6wn5b8yx66hpagd-next-gtk-webkit-0.0.0/share\
:/gnu/store/6l1qcw9731psvp4qp28dvq9j2g1nbnmm-sbcl-trivial-features-0.8/share\
:/gnu/store/g82q0i0wah4d2qh012yi793rvg526s59-sbcl-trivial-garbage-0.21/share\
:/gnu/store/x8k2la7c69dfr62vrgijajp81hybd79s-sbcl-alexandria-0.0.0-1.926a066/share\
:/gnu/store/jdnsak2ylxshg27cxr9fh9gbx1zpwzar-sbcl-bordeaux-threads-0.8.6-1.5dce49f/share\
:/gnu/store/52qzpiy63l6yy9n0icryvf317hiwiiyq-sbcl-cl-css-0.1-1.8fe654c/share\
:/gnu/store/sbhkpri9avslvsmzv0fahswbp9g4igy3-sbcl-cl-json-0.5-1.6dfebb9/share\
:/gnu/store/44dsabdh9fn1j4m9gcll5lqmrnycss9z-sbcl-cl-markup-0.1-1.e0eb7de/share\
:/gnu/store/99ynic4nmhq4mxbrj1rx66lvbw3gki01-sbcl-cl-ppcre-2.0.11/share\
:/gnu/store/ksxf4v32xcmhw3vnrhq8rkyvzwgfx7vk-sbcl-cl-ppcre-unicode-2.0.11/share\
:/gnu/store/ry9pmrqyxgyva7l5747g7ip6mgwkksys-sbcl-cl-string-match-0-1.5048480/share\
:/gnu/store/1gsm5f75rv4isdylbzrkvycw2gh2969h-sbcl-cl-strings-0.0.0-1.c5c5cba/share\
:/gnu/store/zpv26q5jgsr7hbgp3rb3nxhkcl1g6r5z-sbcl-closer-mop-1.0.0-1.fac29ce/share\
:/gnu/store/k1m8m4wxkvqx354h0f48650a57c4ivgk-cl-dbus-20190408-1.24b452d/share\
:/gnu/store/axfsl7653s96cicv7rx1rm26amxjyalr-sbcl-dexador-0.9.10-1.a2714d1/share\
:/gnu/store/3wbyra9x6qjlfp2p3ijxpnlz5pn33xpw-sbcl-ironclad-0.42/share\
...
--8<---------------cut here---------------end--------------->8---

I think the XDG_DATA_DIRS is set automatically from the set of direct inputs,
but it's does not include the recursive inputs.

For asdf-build-system/source to work properly, we would need to force
the input.

Actually, asdf.scm has this comment:

--8<---------------cut here---------------start------------->8---
      ;; Special considerations for source packages: CL inputs become
      ;; propagated, and un-handled arguments are removed.
--8<---------------cut here---------------end--------------->8---

But I think that propagating the inputs might not suffice to build other
packages.

Thoughts?

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