Merge develop for v0.8.16 release.

lib/spack/docs/site_configuration.rst

lib/spack/env/clang

lib/spack/env/clang++

lib/spack/env/g++

lib/spack/env/gcc

lib/spack/llnl/util/filesystem.py

lib/spack/llnl/util/link_tree.py

lib/spack/spack/build_environment.py

lib/spack/spack/package.py

var/spack/packages/SAMRAI/package.py

var/spack/packages/cram/package.py

var/spack/packages/metis/package.py

var/spack/packages/mpich/package.py

var/spack/packages/muster/package.py

var/spack/packages/otf/package.py

var/spack/packages/ravel/package.py

var/spack/packages/scotch/package.py