Hi!

Maxim Cournoyer <maxim.cournoyer@gmail.com> skribis:

> I was surprised that:
>
> guix build fpc --system=armhf-linux
>
> would send me in a dead end without any word of caution, given that
> armhf-linux is *not* listed in the supported-systems field.
>
> We should at least warn near the beginning of the build, and remind of
> that fact at the end of the build in case of failure.

I came up with the patches below.

The first part forbids unsupported packages altogether in user-facing
commands: ‘guix install’, ‘guix shell’, etc.  Until now, they’d just
start building things that were bound to fail.

The second part adds a mere warning for ‘guix build’ (it does not repeat
it at the end, though).

Thanks,
Ludo’.