Reinstallable lib:GHC

We can avoid the need for a configure script for ghc by using the headers shipped along with the rts package, as obtained using ghc-pkg field rts include-dirs.

This is OK since we will only support reinstalling the same version of GHC as has been originally installed. Also, a reinstallable rts is not presently in scope.

For cross compilation purposes, ghc-pkg field rts ... just needs to point to the rts library for the target platform.

TODO:

• upload deriveConstants and genprimopcode to hackage
• Use bundled rts includes instead of a relative include to ../includes/rts/... in GHC.