Conditional javadocs build

[Mark Hewitt]

Is it possible to tell the javadoc task to execute only if there 
have been any changes to the packages over which it is running?

For some of our projects, the resources required to generate the
javadocs are rate limiting, and while I can conditionalise the 
compilation, there does not seem to be a well defined means of
doing the same for javadoc.

#!/mjh