Issues with Haskell's ghc

[Vasilii Kolobkov]

Hello guix!

I've run aground with Haskell's ghc (interpreter works fine) - running
ghc on a basic hello world program fails.  I identified couple issues:

 - Include directory for linux headers wasn't passed to gcc
   (`C_INCLUDE_PATH=/gnu/... ghc` fixed it)
 - Lib C's crti.o and crt1.o where not found during linking
   (fixed with `LIBRARY_PATH=/gnu/... ghc`)
 - libgmp and libffi aren't passed to linker's -rpath

If anybody uses Haskell here, i'd appreciate any insights on what might
be the root cause here.

Thanks!