bug#51213: ghci missing reference to gcc

[Jack Hill <jackhill@jackhill.us>]

On Thu, 14 Oct 2021, Jack Hill wrote:

> Hi Guix,
>
> Using Guix 5fafb6e792c1e18ec223f0c7fb350d0c9b962a45, running ghci in an 
> environment can't find gcc:

Some more observations about this problem. It only occurs with our two 
newest GHC packages (8.10.7 and 8.8.4), previous releases start ghci and 
perform light usage fine. However, all releases fail to build executables 
without gcc-toolchain in the path:

```
jackhill@alperton /tmp [env]$ ghc hello.hs 
[1 of 1] Compiling Main             ( hello.hs, hello.o )

<no location info>: error:
     Warning: Couldn't figure out C compiler information!
              Make sure you're using GNU gcc, or clang
ghc: could not execute: gcc
jackhill@alperton /tmp [env]$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 8.10.7
```

Interestingly, clang-toolchain can be substituted for gcc-toolchain to get 
a working ghci, but with 8.10.7, compiling with gcc still doesn't work, 
but ghc 7.10.2 find clang-toolchain to be satisfactory.

Best,
Jack