Use cstdatomic header in newer GCC

Starting wtih GCC 4.4, the atomic implementation from the upcoming C++ standard is being shipped. Rather than continuing to carry our own atomics implementation in tree, we should begin using this. However, there will be a transition period where not everyone has either GCC 4.4 or a standard implementation. To address this, we will:
a) Modify the interface of our atomic<> class to match the standard if we deviate from it
b) Put in an autoconf test for the existence of cstdatomic header
c) Have our header just use cstdatomic if it exists, If it does not exist, use the in-tree implementation.