Add support for using GHC's -jsem option

* The top-level user interface to enable the use of the semaphore is the
  `--semaphore` flag. If you pass `-j4 --semaphore` then cabal will
  create a semaphore with 4 slots which is passed to ghc using the
  `-jsem` option so that all GHC processes coordinate to use 4
  capabilities.

  - The semaphore logic is provided by a new package `semaphore-compat`
    which provides a cross-platform abstraction for semaphores.

* The low level `./Setup.hs build` interface accepts the `--semaphore
  <SEMAPHORE>` option, which can be used to directly pass the semaphore
  you require to the Setup script.