Compiling setup does not honor the --with-gcc flag

I am using stack 2.1.3 on Linux x86_64, downloaded from the Stack website.

I am in an environment where the configured `gcc` doesn't exist, so I'm passing `stack install --with-gcc=/special/gcc`. Unfortunately I get the error:

```
[1 of 2] Compiling Main ( /home/me/.stack/setup-exe-src/setup-mPHDzAJ.hs, /home/me/.stack/setup-exe-src/setup-mPHDZzAJ.o)

: error:
Warning: Couldn't figure out C compiler information!
Make sure you're using GNU gcc, or clang
ghc: could not execute: /garbage/path/where/ghc/thinks/its/settings/are/bin/../lib/../../../../../gcc/stuff/bin/gcc

-- While building simple Setup.hs using:
/system/ghc -rtsopts -threaded -clear-package-db -global-package-db -hide-all-packages -package base -main-is StackSetupShim.mainOverride -package Cabal-2.2.0.1 /home/me/.stack/setup-exe-src/setup-mPHDZzAJ.js /home/me/.stack/setup-exe-src/setup-shim-mPHDZzAJ.hs -o /home/me/.stack/setup-exe-cache/x86_64-linux/tmp-Cabal-simple_mPHDZzAJ_2.2.0.1_ghc-8.4.4
Process exited with code: ExitFailure 1
```

My hypothesis is you are forgetting to pass the explicit path to gcc when building Setup.hs?