Disable the new -Wunused-but-set-variable that was added in gcc 4.6.0
It produces more false positives than useful warnings.

This can still be enabled using W=1
Signed-off-by: NDave Jones <davej@redhat.com>
Acked-by: NSam Ravnborg <sam@ravnborg.org>
Tested-by: NSam Ravnborg <sam@ravnborg.org>
Signed-off-by: NMichal Marek <mmarek@suse.cz>