MacOS requires c++ flag (#1000)

Summary:
To install on MacOS, `-stdlib=libc++` needs to be specified.
Pull Request resolved: https://github.com/pytorch/fairseq/pull/1000

Differential Revision: D16733819

Pulled By: myleott

fbshipit-source-id: 7a1ed11e2b4e1071e61c64c379c84f72e02ad2b5

README.md

@@ -62,7 +62,10 @@ After PyTorch is installed, you can install fairseq with `pip`:
 ```
 pip install fairseq
 ```
-
+On MacOS,
+```
+CFLAGS="-stdlib=libc++" pip install fairseq
+```
 **Installing from source**
 
 To install fairseq from source and develop locally: