ENABLE_TREE_CHECKING

[Alexandre Petit-Bianco]

Without any real warning, someone decided to let build the Java
front-end with ENABLE_TREE_CHECKING turned on. Not a bad thing to do,
except that we were totally unprepared to this transition. Libgcj
doesn't build anymore with the latest compiler.

It's been like this the past three days -- sorry for not trying to
address the situation earlier, but I've been basically out of reach
this week.

./A