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

lib/spack/docs/packaging_guide.rst

lib/spack/env/cc

lib/spack/spack/concretize.py

lib/spack/spack/directives.py

lib/spack/spack/fetch_strategy.py

lib/spack/spack/patch.py

lib/spack/spack/stage.py

lib/spack/spack/test/versions.py

lib/spack/spack/util/compression.py

lib/spack/spack/version.py

...ck/repos/builtin.mock/packages/externalvirtual/package.py

...spack/repos/builtin.mock/packages/othervirtual/package.py

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

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

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

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

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

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

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

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