Resurrect UseLibFFIForAdjustors from bitrot.
* Pass -Irts/dist/build to the C preprocessor to expose libffi headers (ffi.h and ffitarget.h) to foreign import wrappers during the building process of GHC itself. * Install libffi headers into $(ghcheaderdir) just like any other C headers. Otherwise an installed GHC can't find them when it wants to compile foreign import wrappers. * Include libffi headers in the bindist for the same reason.