Add feature to generate cabal_macros.h

We would like to generate and use `cabal_macros.h` (e.g. for `hlint` or our `ghc -M` based build system) without actually running `cabal build`.

I was surprised that `cabal_macros.h` is created by `cabal build` only, and not by `cabal configure`.

Also, while you can do `cabal build mytarget`, there isn't an empty target, so there currently is no way to tell cabal to _only_ generate `cabal_macros.h` and do no further compilation.

It would be great to have a way to generate only `cabal_macros.h`; maybe via one of these alternatives:
- make it part of `cabal configure`
- add a flag `cabal build`
- add an empty build target in some way (maybe `cabal build ""`)