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

[Mark Mitchell]

This patch is OK for the mainline and the branch.

I'm not sure where any of the top-level configury is documented.  I
think we should document these options, but I'm not sure where.  I
think Gerald will know, though, so I've copied him as well.  Please
install appropriate documentation, once we know where.

Thanks,

--
Mark Mitchell                   mark@codesourcery.com
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