Merge branch 'develop' of https://github.com/LLNL/spack into openmpi_variants

lib/spack/docs/packaging_guide.rst

lib/spack/spack/compiler.py

lib/spack/spack/compilers/clang.py

lib/spack/spack/compilers/gcc.py

lib/spack/spack/compilers/intel.py

lib/spack/spack/compilers/nag.py

lib/spack/spack/compilers/pgi.py

lib/spack/spack/compilers/xl.py

var/spack/repos/builtin/packages/kripke/package.py