On Cray machines, use the Cray compile wrappers instead of MPI wrappers.

+ Cray compile wrappers are MPI wrappers.
+ Packages that need to be compiled with MPI compile wrappers normally use
  'mpicc', 'mpic++' and 'mpif90' provided by the MPI vendor. However, when using
  cray-mpich as the MPI vendor, the compile wrappers 'CC', 'cc' and 'ftn' must
  be used.
+ In this scenario, the mpich package is hijacked by specifying cray-mpich as an
  external package under the 'mpich:' section of packages.yaml. For example:

  packages:
    mpich:
      modules:
        mpich@7.4.2%intel@16.0.3 arch=cray-CNL-haswell: cray-mpich/7.4.2
      buildable: False
    all:
      providers:
        mpi: [mpich]

+ This change allows packages like parmetis to be built using the Cray compile
  wrappers. For example: 'spack install parmetis%intel@16.0.3 ^mpich@7.4.2 os=CNL'
+ This commit relies on the existence of the environment variable CRAYPE_VERSION
  to determine if the current machine is running a Cray environment. This check is
  insufficient, but I'm not sure how to improve this logic.
+ Fixes #1827

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