For the time being could we have something like?

```
JRUBY_CMD?=jruby
cd ruby && $JRUBY_CMD -S rake
```

This can then be overridden where necessary, e.g.

```sh
JRUBY_CMD='RBENV_VERSION=`rbenv versions | grep jruby | tail -1 | tr -d " "` jruby' make
```