Install gnu-getopt on Mac OS

Install gnu-getopt (with Homebrew) and configure shflags to use it:

$ brew install gnu-getopt $ echo 'export FLAGS_GETOPT_CMD="$(brew --prefix gnu-getopt)/bin/getopt"' >> ~/.bashrc

Start a new terminal session to make sure your login shell picked up FLAGS_GETOPT_CMD:

```
$ echo $FLAGS_GETOPT_CMD
```

which should return something like: /usr/local/Cellar/gnu- getopt/1.1.4/bin/getopt