fix OCaml for ninja-build generator

```bash
$ cmake -G Ninja .
$ ninja -j4
```
![2014-07-08 01 49 51](https://cloud.githubusercontent.com/assets/3759759/3503335/b535d782-062e-11e4-87b7-ff98c4df1bca.png)

@fredemmott XCode/Ninja `$(MAKE)` undefined.