Hi Ben,

I observed warnings of this kind in a couple of places building head yesterday:
```
compiler/utils/FastString.hs:336:5: error:
     warning: 'GHC_STAGE' is not defined, evaluates to 0 [-Wundef]
    |
336 | #if GHC_STAGE < 2
    |     ^
#if GHC_STAGE < 2
    ^
```

```
compiler/main/DynFlags.hs:323:5: error:
     warning: 'GHC_STAGE' is not defined, evaluates to 0 [-Wundef]
    |
323 | #if GHC_STAGE >= 2
    |     ^
#if GHC_STAGE >= 2
    ^
```
I was using the `hadrian/build.stack.sh` approach. Figured you'd want to know!