erlc-include: echo -n does not work under OS X

When running `make` using erlang.mk 1.1.0, the output looks something like this:

```bash
-n
APP    noesis.app.src
```

The cause of this is the `echo -n` in the `erlc-include` target.

Apple mentions this in the [man pages](https://developer.apple.com/library/mac/documentation/Darwin/Reference/ManPages/man1/echo.1.html). Changing the line to `/bin/echo -n` works, but I'm not sure if that's a good idea.