v0.15.1 (2020-07-10)

This minor release includes several important fixes:

• Fix shell support on Cray (#17386)
• Fix use of externals installed with other Spack instances (#16954)
• Fix gcc+binutils build (#9024)
• Fixes for usage of intel-mpi (#17378 and #17382)
• Fixes to Autotools config.guess detection (#17333 and #17356)
• Update spack install message to prompt user when an environment is not explicitly activated (#17454)

This release also adds a mirror for all sources that are fetched in Spack (#17077). It is expected to be useful when the official website for a Spack package is unavailable.