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

[Vincent Habchi]

Ryan,

> another compiler just to build a port if I can avoid it. However, atlas seems to be a special case, and if performance is the most important thing for atlas, and clang achieves best performance, and if your users won't mind having (to wait for MacPorts) to install another compiler to get it, then go ahead and make clang-3.3 the default variant.

The purpose of Atlas is precisely to provide the best computing performance. The code tries over and over dozens of different assembly snippets, plays with cache lines, prefetch lengths, inner loops and so on to achieve maximum power. It would not only be a waste, but even an insult against Atlas philosophy not to allow it to be compiled with the most efficient optimizations.

Vincent