Fix path to JavaFX to build on Mac OS.

I don't know why there should be a platform dependency. I have not checked other platforms. I am running Mac OS X Mountain Lion and Java 7:

``` console
$ java -version
java version "1.7.0_21"
Java(TM) SE Runtime Environment (build 1.7.0_21-b12)
Java HotSpot(TM) 64-Bit Server VM (build 23.21-b01, mixed mode)
```