Atomic issues

Recent compilers seem to have an issue with [atomic](http://www.cplusplus.com/reference/atomic/) : 
```
/home/alexv/Projects/tinythreadpp/test/test.cpp:51:1: error: reference to ‘atomic_flag’ is ambiguous
 atomic_flag gFlag(ATOMIC_FLAG_INIT);

```

Indeed  `atomic_flag` is redefined in `tinythread.h`, even if it might not be necessary (already included in standard `atomic_base.h`) 