-Wnon-virtual-dtor is part of -Wall on gcc 4.0.3, but not on 4.2.4.
Enable it specifically.

Review URL: http://codereview.chromium.org/160450

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@2600 ce2b1a6d-e550-0410-aec6-3dcde31c8c00