Show message box on assert only if USE_MSBOX is true; reviewed as http://codereview.appspot.com/1668044
Show message box on assert only if USE_MSBOX is true; reviewed as http://codereview.appspot.com/1668044

git-svn-id: fe3cd4388a6e5a968b09397a3d721fe124e8117a@19 5b54908e-ead4-fc18-b48a-0285e6dbc711

rnk/drmemory,rnk/drmemory,rnk/drmemory,rnk/drmemory