Summary:
When we build boost 1.69.0 with newer version of Xcode, it will fail with:

```
clang: error: unknown argument: '-fcoalesce-templates'
```

This commit fixes this build failure by telling boost's build system that we are building with clang on macOS.

Reviewed By: chadaustin

Differential Revision: D22417488

fbshipit-source-id: 0b3d22835abbba6d06812c656acb0311a60d8c67