Patch to enable libgcj.dll for MinGW

[TJ Laurenzo]

> Oh dear, I am *really* sorry that making this change is causing so
> much trouble!  Thanks for your patience.
> 
> ATTRIBUTE_UNUSED does the job, and you'll find examples of its use in
> gcc sources.

Thanks to everyone who participated in this lengthy discussion.  I'll
incorporate ATTRIBUTE_UNUSED and submit the patch.  No worries about
the extra work.  I had the time to slog through it this time, and I
learned quite a few things in the process.

TJ