Build script error

When running the install script `install-ndk.sh` I get the error

```
/usr/bin/readlink
readlink: illegal option -f
```
I believe this is because `readlink` does not exist on Mac systems, as mentioned in [this stackoverflow](https://stackoverflow.com/questions/1055671/how-can-i-get-the-behavior-of-gnus-readlink-f-on-a-mac).