Re: Build Mesa with a different toolchain version

[Thiago Jung Bauermann <bauermann@kolabnow.com>]

After sending the email, I thought of another thing to try, but it didn't
help. I thought I'd mention it here for completeness:

Try #3:

Using this definition in my local channel:

```
(define gcc->gcc-10
  (package-input-rewriting `((,gcc-7 . ,gcc-10)
                             (,(make-libstdc++ gcc-7) . ,(make-libstdc++ gcc-10)))))

(define-public mesa-10
  (package
    (inherit (gcc->gcc-10 mesa))
    (name "mesa-10")))
```

i.e., the only change relative to the previous try was to look for ‘gcc-7’
rather than ‘gcc’ to make the substitution.

But unfortunately guix build didn't even try to rebuild the package.
It just returned the store items that were built in try #2:

$ guix build --system=i686-linux mesa-10
/gnu/store/8iiwn9cnjlfxfi7m1fmdavzgr7y38vv7-mesa-10-20.2.4-bin
/gnu/store/hjci6cqw76hvdawdl04kjmbajklsdwx2-mesa-10-20.2.4

I believe this shows that ‘gcc’ already points to ‘gcc-7’. I just can't
change it for some reason…

-- 
Thanks,
Thiago