ghc-7.8.4: re-implement "fix build on Darwin" to avoid re-builds on Linux

Originally, I thought that I can commit a "clean" patch -- even if it
triggers re-builds -- because those re-builds were triggered by the
ncurses patch to GHC anyway . That patch had to be reverted, though, so
now I'm rewriting this patch to avoid re-builds on Linux.

What a mess. :-(