test: silence warnings on clang under clang-cl
test: silence warnings on clang under clang-cl

When running under clang-cl mode, we do not define `__GNUC__`, resulting
in the test failing.

git-svn-id: 756ef344af921d95d562d9e9f9389127a89a6314@295560 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx