Use new gcc flag -Wno-unused-but-set-variable only on linux.
TBR=fschneider@chromium.org

BUG=
TEST=

Review URL: http://codereview.chromium.org/7457018

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@8689 ce2b1a6d-e550-0410-aec6-3dcde31c8c00