Use ATLAS as the example BLAS in building.rst

OpenBLAS has subtle issues releated to threading. It
conflicts with the use of threads in the other parts of
the application.

Careful users can still use it by disabling threads via
an environment variable, but by default we want to use
a BLAS/LAPACK that does not suffer from these problems.

Change-Id: I8c1c0ed0b526453564c5f9ea69b646fac32fe027
1 file changed