Run ant headless so no dock icon will appear

It not only appears in the dock, but *because* it appears in the dock it
steals focus from whatever you are doing at the time.

Specifying `java.awt.headless=true` as a JVM arg stops this annoying behavior.

q.v. https://discussions.apple.com/message/6052815#6052815

I tested this on Windows to ensure that it still functions as designed, and it worked for me.