ensure T_GLOBAL_DEBUG_VIRTUAL is defined (-Wundef)
ensure T_GLOBAL_DEBUG_VIRTUAL is defined (-Wundef)

Summary: We'd like to build code in ovrsource with `-Wundef`.

Reviewed By: yfeldblum

Differential Revision: D16856736

fbshipit-source-id: 0943ef2807dfb6a22fcbfb3a0d4ef595ded35798

facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift