Allow 'if' for data-files section

[augustss]

In the MicroHs cabal file I'd like to say this

data-files:
      src/runtime/*.c
      src/runtime/*.h
if impl(ghc)
   data-files:
      lib/**/*.hs
      lib/**/*.hs-boot

But when I tried it, it didn't work. It works in MicroCabal so I've managed to hack around it, but it seems like an obvious thing that should work.

[ffaf1]

What is cabal sdist supposed to do? Pack all of those files or conditionally exclude some?

[augustss]

I'd expect cabal sdist to include all the files so it can do the the right thing when you later do cabal install of that package.