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.

(cherry picked from commit 0a1c167a)

# Conflicts:
#	Cabal/src/Distribution/Simple/Build.hs
#	Cabal/src/Distribution/Simple/Compiler.hs
#	Cabal/src/Distribution/Simple/GHC.hs
#	Cabal/src/Distribution/Simple/GHCJS.hs
#	Cabal/src/Distribution/Simple/Program/GHC.hs
#	Cabal/src/Distribution/Simple/Setup.hs
#	Cabal/src/Distribution/Simple/Setup/Build.hs
#	bootstrap/linux-9.4.5.json
#	cabal-install/src/Distribution/Client/Config.hs
#	cabal-install/src/Distribution/Client/JobControl.hs
#	cabal-install/src/Distribution/Client/ProjectBuilding.hs
#	cabal-install/src/Distribution/Client/ProjectConfig.hs
#	cabal-install/src/Distribution/Client/ProjectConfig/Legacy.hs
#	cabal-install/src/Distribution/Client/ProjectConfig/Types.hs
#	cabal-install/src/Distribution/Client/ProjectPlanning.hs
#	cabal-install/src/Distribution/Client/Setup.hs
#	cabal-install/tests/UnitTests/Distribution/Client/ProjectConfig.hs