Changing default compiler for Atlas from gcc47 to clang-3.3 on AVX capable machines

[Vincent Habchi]

Ryan,

> But atlas also uses fortran, right? Is it more important for atlas to have better performance by compiling with clang, or to use fortran by compiling with gcc?

Atlas uses Fortran only to build the BLAS/Lapack fortran API. The Fortran compiler performance is irrelevant to the whole performance, and Atlas can also be built without Fortran (but, of course, you only get the C API).

The new Portfile clamps the optimization flag of gfortran to -sse4.2 in order to avoid AVX instruction generation by gfortran47 or above.

Vincent