Explicitly support building with -fno-code

**Summary by @ezyang.** There is no way to ask Setup to just typecheck a package. It sort of works if you pass `-fno-code` via `--ghc-options` but that's purely accidental.

---

I use `cabal build --ghc-options="-Wall -fno-code"` to quickly get all errors/warnings from a project and show them in an editor.

However, when I have something like this

```
executable Shared.so
hs-source-dirs:
apps
main-is:
Shared.hs
build-depends:
base >= 4 && <= 5
ghc-options:
-optl-shared -optc-DMODULE=Shared -no-hs-main -fPIC -shared -dynamic
cc-options: -DMODULE=Shared -shared
ld-options: -shared /home/niklas/opt/haskell-7.4/lib/ghc-7.4.2/libHSrts-ghc7.4.2.so
```

in my cabal file that only builds a shared object, I get:

```
Warning: the following files would be used as linker inputs, but linking is not being done: dist/build/...
ghc: no input files
Usage: For basic information, try the `--help' option.
```

Should we get this warning although we explicitly said `-fno-code`?

If yes, can we add an option to disable this kind of warning so that it does not clutter the warnings I am actually interested in (to show in the editor)?

---

**Update:**

(The original title was _Add option to disable non-linking warnings with -fno-code_.)

I just realized that there is an actual bug in this:

After the warning, due to the ghc _error_ cabal will just **stop** and build no further. This should not happen as there is no code to build!