> 6. Good support for building against external packages that may already be on your system. Able to use 'internal' packages (spack install openssl) or externa

> 6. Good support for building against external packages that may already be on your system. Able to use 'internal' packages (spack install openssl) or external packages (link against /usr/lib/libssl.so) on a per item basis: sometimes you want the newest code, but other times you want the OS package so that you can get security updates 'for free'. Most other system do one or the other: Spack allows you to choose.