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.
Before this change the `ghcjs` build fails with:
```
checking host system type... Invalid configuration `js-ghcjs': machine `js-unknown' not recognized
configure: error: …/bin/bash …/configured-ghcjs-src/lib/boot/pkg/unix/config.sub js-ghcjs failed
```
This fails because the `updateAutotoolsGnuConfigScriptsHook` overrides the
`config.sub` in the `unix` boot package to use the newer upstream
version of `config.sub`, but the newer version is incompatible with the
`js-ghcjs` host string used to configure the `unix` package. The
fix is to undo the override, because the `config.sub` vendored within
`ghc` supports the host string correctly.
