check whether -Wno-unused-but-set-variable is supported since gcc 4.1.2 aborts if not
check whether -Wno-unused-but-set-variable is supported since gcc 4.1.2 aborts if not

git-svn-id: ed337ca7cdb08c0ba29d8fe58632bb7686892c2d@832 49cc7528-f6fd-11dd-9d1a-b59b2e1864b6

rnk/dynamorio,rnk/dynamorio,rnk/dynamorio,rnk/dynamorio,rnk/dynamorio