[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

Author: stepancheg

caffe2/perfkernels/cvtsh_ss_bugfix.h

@@ -2,15 +2,15 @@
 
 // Apple clang was fixed in 8.1
 #if defined(__apple_build_version__) && ((__clang_major__ < 8) || ((__clang_major__ == 8) && (__clang_minor__ < 1)))
-#define __APPLE_NEED_FIX 1
+#define CAFFE2_INTERNAL_APPLE_NEED_FIX 1
 #endif
 
 // Regular clang was fixed in 3.9
 #if defined(__clang__) && (__clang_major__ < 4) && (__clang_minor__ < 9)
-#define __CLANG_NEED_FIX 1
+#define CAFFE2_INTERNAL_CLANG_NEED_FIX 1
 #endif
 
-#if __APPLE_NEED_FIX || __CLANG_NEED_FIX
+#if defined(CAFFE2_INTERNAL_APPLE_NEED_FIX) || defined(CAFFE2_INTERNAL_CLANG_NEED_FIX)
 
 #include <c10/util/Half.h>
 #include <emmintrin.h>