Support hsc2hs preprocessor

[mboes]

Not all source files can be handed over to as-is to GHC. hsc2hs is one such preprocessor that needs to be called by hand. Cabal in fact defines a number of such hard-coded preprocessors: happy and alex among them. I'm not too worried about those, which are easy for the user to special-case using a genrule. But hsc2hs is a tad tricky.

It sometimes creates multiple output files, sometimes not. At most, these output files are (given an input file A.hsc): A.hs, A_hsc.h, A_hsc.c. Other complications include the fact that the same -I and possibly even -L and -l compiler switches than for ghc need to be passed to hsc2hs as well.

We need a rule of the following form:

haskell_hsc(
  name = "foo",
  src = "Foo.hsc",
)

[mboes]

Fixed as a byproduct of #14.