[caffe2] Fix -Wundef
Summary:
* `#if` with some undefined name is a warning when `-Wundef` is specified (which is in ovrsource for example)
* identifiers starting with two underscores are [reserved for compiler internals](https://en.cppreference.com/w/cpp/language/identifiers)
Test Plan: CI
Reviewed By: ezyang
Differential Revision: D27318070
fbshipit-source-id: 4989fc6a3bf3c176eddd7c25aca47414e4973edd
