Explicitly pass CXX to NCCL Makefile
Explicitly pass CXX to NCCL Makefile

Summary:
Necessary if CXX isn't set when cmake is called. The CXX variable will then be
empty which prevents make from using its own default.
Closes https://github.com/caffe2/caffe2/pull/202

Differential Revision: D4711113

Pulled By: Yangqing

fbshipit-source-id: 895c07044b263ba9b5440453978248506d7ac225

Yangqing/caffe2,xzturn/caffe2,Yangqing/caffe2,sf-wind/caffe2,pietern/caffe2,bwasti/caffe2,sf-wind/caffe2,davinwang/caffe2,bwasti/caffe2,bwasti/caffe2,bwasti/caffe2,pietern/caffe2,bwasti/caffe2,Yangqing/caffe2,davinwang/caffe2,sf-wind/caffe2,pietern/caffe2,xzturn/caffe2,Yangqing/caffe2,pietern/caffe2,pietern/caffe2,davinwang/caffe2,davinwang/caffe2,sf-wind/caffe2,Yangqing/caffe2,xzturn/caffe2,sf-wind/caffe2,xzturn/caffe2,davinwang/caffe2,xzturn/caffe2,caffe2/caffe2