Switch -Wunused-variable on?

[Sergio Durigan Junior]

Hello,

While doing something else, I decided to enable the -Wunused-variable
option and... bummer.  So I decided to prepare a patch to fix this,
because there are *many* places with unused variables.

However, before continuing to prepare this patch, I would like what you
think about enabling -Wunused-variable by default in the building.  I am
not sure I can foresee something wrong with it, but maybe others have
some different opinions.

Thanks,

-- 
Sergio

P.S.: I couldn't find any real discussion about this on the list
archives, but please excuse me if this has been already discussed.