Use system ATLAS

[embray]

We can avoid building ATLAS as part of the Sage build if we install the system version and set some environment variables at Sage build time.

The ATLAS included in the APT repository is generic and not specifically compiled for any one CPU--this makes sense of course, and is our only option anyways for the sake of a reusable container. If anyone with more experience getting the bleeding-edge performance out of ATLAS wants to chime in though that would be helpful.

[nthiery]

Just a note: this might not be critical anymore with the switch to openblas?