This change was made in JDK 7 b19 or so, and is also in open jdk 6.
It creates these files:

<jdk>/demo/jpda/
src.zip -- source code for jpda examples
jpda.jar -- .class files for jpda examples

Previously, this Makefile just created examples.jar which contained
the source code for the jpda examples.

It seems to me that we should undo this change because:
- the doc bundle refers to examples.jar
- users might be used to seeing examples.jar
- maybe we should not include dependcies on outside utilities
  such as zip in the JDK?
- it doesn't seem worth wasting bandwidth and disk space
  for the .class files when very few people actually use these examples.