More info:

https://gitlab.kitware.com/cmake/cmake/-/issues/20568

https://gitlab.kitware.com/utils/kwsys/-/merge_requests/187

> Furthermore, I'd like to ask: Why is json-c a dependency in the first place ?

Because of sdl2.  I've removed sdl2 from the native-inputs of u-boot
and added it to the native-inputs of u-boot-tools in guix master
commit 6b1253718d1d660e7a91bd59a96bdb16d7c5e0b3.

So now only this fails:

$ guix build -s armhf-linux u-boot-tools