message: Explicitly define XlaCompilationCache::kDefaultCompilationThreshold.
  clang 4.0.1-10 thinks this variable is ODR-used and so raises an error that there's no definition.
diff: @@ -38,6 +38,8 @@ limitations under the License.
  namespace tensorflow {
  +constexpr int64 XlaCompilationCache::kDefaultCompilationThreshold;
  +
  XlaCompilationCache::XlaCompilationCache(xla::LocalClient* client,
  DeviceType device_type)
  : client_(client), device_type_(std::move(device_type)) {}
  