[v3] mutex vs. cstdatomic

[Paolo Carlini]

Benjamin Kosnik wrote:
> Here's the fix, in at revision 142742.
>   
Thanks. However, I didn't notice before that, because of that
static_assert bug, we are now including <cassert>. Really, we don't want
that, remember when we added __replacement_assert for debug-mode...
Either we can temporarily do the same here, or workaround the
static_assert issue directly, I don't know...

Paolo.