Unstable `lockfree boost::atomic_flag` capability on MacOS Mojave over rebuild

```
./bootstrap.sh --prefix=/work/boost/build --with-libraries=context
./b2
Performing configuration checks

- default address-model : 64-bit
- default architecture : x86

Building the Boost C++ Libraries.

- symlinks supported : yes
- C++11 mutex : no
- lockfree boost::atomic_flag : no
...
```
then remove `./bin.v2` folder and try again with b2:

```
./b2
Performing configuration checks

- default address-model : 64-bit
- default architecture : x86

Building the Boost C++ Libraries.

- symlinks supported : yes
- C++11 mutex : no
- lockfree boost::atomic_flag : yes
```

Note that `lockfree boost::atomic_flag` became 'yes' on second pass.