[PATCH, libjava/classpath]: Fix overriding recipe for target 'gjdoc' build warning

[Tom Tromey]

Jeff> In the past this has stalled on issues like how will asynch-exceptions
Jeff> be tested and the like.

It seems to me that either there is some other language which needs this
-- in which case that language ought to have testing for the feature --
or the feature is only used by gcj, in which case it doesn't matter.

Of course is!=ought; but relying on gcj and libjava to provide this
small amount of testing seems like a bad cost/benefit tradeoff.

Tom