bug#39794: AVR-Toolchain-5 avr/io not found

[Ekaitz Zarraga <ekaitz@elenq.tech>]

> > I can replicate what you did and it works.
>
> Nice! :-)
>
> > May it be conflicting with other packages?
>
> arm-none-eabi-toolchain is conflicting with avr-toolchain since both are
> setting the CROSS_* environment variables. See the output of `avr-gcc -E -v -` below.
>
> This should probably be considered a bug. But I have no idea how to fix
> this. :-( What do you think?

I agree. The problem I see here is the environment variable game we have here.
One option is to create cross compilers with the include paths inserted on them instead of taking environment variables as a default.

If they had the correct directories in their default includepaths everything would work correctly.

I never did this before but it should be possible, probably affecting cross-gcc's configure step.

> Also, our avr-gcc package should probably be updated to the latest
> avr-gcc-9.

Once the current fix is applied next fixes should be possible.

Why wasn't the patch included in the project?
We can start by pushing it and then keep solving the conflict issue and the upgrades to gcc-9.

Have to say that nature of Guix makes the conflict issue easy to avoid while we find a solution for it.