[caffe2] Use the CXX11 version of the USE_C99_COMPLEX macro
Summary: Because the bare CXX version forwards to this without checking if it's defined causing errors for builds with -Wundef enabled
Test Plan: contbuilds
Differential Revision: D27443462
fbshipit-source-id: 554a3c653aae14d19e35038ba000cf5330e6d679
