Defer enabling std::atomic on GCC until GCC 4.7

The implementation that GCC came with from 4.4 to 4.6 used external
locks for most platforms, other than x86. It's unsuitable to be called
"atomic" at all. More importantly, it's not behaviour-compatible with
Qt's own QBasicAtomic classes.

Change-Id: I92a0beab58504e6bd7cd236d358fef905c69821c
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>