Limit this fix to apple clang only
Limit this fix to apple clang only

Summary:
Use "__apple_build_version__" macro to distinguish Apple's Clang while brew installed LLVM will compile caffe2 without trouble.
Closes https://github.com/caffe2/caffe2/pull/1461

Differential Revision: D6316861

Pulled By: Yangqing

fbshipit-source-id: f7a08cdd8822b197a93aa11dc8f28ef5cd738eee

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