Fixes#2781
This PR introduces a new attribute for packages called
`archive_files`, which designates files that should be saved from
a package build (e.g. the config.log generated during autotools
builds).
The attribute contains a list of glob expressions; Any file that
matches will be archived in the `<prefix>/.spack/archived-files`
directory. Errors that occur when archiving files are collected and
reported in a file named `<prefix>/.spack/archived-files/errors.txt`.
`AutotoolsPackage` and `CMakePackage` provide a sensible default
override for this attribute.