Fix building with -DFAISS_OPT_LEVEL=avx2 (#1746)
Fix building with -DFAISS_OPT_LEVEL=avx2 (#1746)

Summary:
As stated in https://github.com/facebookresearch/faiss/issues/1743, `_swigfaiss.so` will not exist if compiled with `-DFAISS_OPT_LEVEL=avx2`. This diff added a conditional statement before copying it.

Pull Request resolved: https://github.com/facebookresearch/faiss/pull/1746

Reviewed By: mdouze

Differential Revision: D26978739

Pulled By: beauby

fbshipit-source-id: 34250c29585fca28677849c9734f6a421661f108

facebookresearch/faiss,facebookresearch/faiss,facebookresearch/faiss,facebookresearch/faiss