cmake: disable -Wnon-virtual-dtor when it gives false positives

clang has only been smart enough not to trigger -Wnon-virtual-dtor
warnings on final classes since r208449 (in clang 3.5). Building
with older versions is extremely noisy, so disable the warning
on those compilers.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216327 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed