Before this change the GHCJS build fails on Linux with:
```
Setup: Missing dependency on a foreign library:
* Missing (or bad) header file: ghc/utils/unlit/fs.h
```
The root cause is that the `./ghc/configure.ac` script is
responsible for copying `./ghc/utils/fs/fs.h` to
`./ghc/utils/unlit/fs.h`, but the script exits early if a C
compiler is not present.
This fixes that by ensuring that the C compiler is present
on all platforms (not just Darwin), so that the build now
works on Linux, too.
