[bug#69790] QA review for 69790

[Parnikkapore]

user guix
usertag 69790 + reviewed-looks-good
thanks

Guix QA review form submission:
/ no new changes that could require the recipe to be modified
/ cross-compilation not affected by current change
/ does not cause a random sample of dependent packages to fail build

Items marked as checked: Lint warnings, Package builds, Commit messages

This fails, but IMO is a Guix bug instead of a bug in this package:
```
$ # --system=x86_64-linux
$ ./pre-inst-env guix build --target=aarch64-linux-gnu fltk
...
ERROR: In procedure %resolve-variable:
Unbound variable: %output
```