RFR: 8185358: gradle 4.X has changed location of built classes

[Phil Race]

In gradle 4 the default location of classes when using has changed from:
Java, `src/main/java` -> build/classes/main
..
to
..
Java, `src/main/java` -> build/classes/java/main

This breaks some parts of the FX build .. which eventually is going to 
need gradle 4.1 for JDK 10
and already does for some testing purposes so I am being bitten by this 
on an almost daily basis.

The fix and a few more details are in-line in the bug :
https://bugs.openjdk.java.net/browse/JDK-8185358

-phil.