Atlas update

[Vincent Habchi]

Folks,

I have kinda spruced up the Atlas port by moving from clang33 to clang34, replacing an old gcc46 variant by a new gcc49, and changing the default behavior (+clang instead of +clang33)

Now, my predicament is this: should I upgrade the revision, and force a general rebuild in all users (given that there is no Atlas binaries and it takes eons to build) or should I just commit the new port in a kinda stealth upgrade?

Thanks for your input!
Vincent