RFR: JDK-8075717 Replace INTERNAL_BUILD with DEBUG in awt

[Magnus Ihse Bursie]

The current define INTERNAL_BUILD has a value that is dependent on the 
milestone part of the version string. This is not ideal, and should be 
replaced with a simple dependency on DEBUG.

Bug: https://bugs.openjdk.java.net/browse/JDK-8075717
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8075717-remove-INTERNAL_BUILD/webrev.01

/Magnus