Support --enable-libgcj and --disable-libgcj

[Alexandre Oliva]

This patch introduces two new flags to the top-level configure.in:

--disable-libgcj: don't build the Java libraries even if the Java
front-end is enabled.

--enable-libgcj: build the Java libraries even if they are not known
to work on the selected target.  (no effect if the Java front-end is
disabled)

I'm not sure we should document these.  If so, where?

Ok to install?