add new build files to gitignore; test that build does not leave git repo checkout dirty (#16565)
Summary:
These appear when I run
```
MACOSX_DEPLOYMENT_TARGET=10.13 CC=clang CXX=clang++ NO_CUDA=1 NO_DISTRIBUTED=1 BUILD_CAFFE2_OPS=0 DEBUG=1 python3 setup.py develop --cmake
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16565
Differential Revision: D13885790
Pulled By: ezyang
fbshipit-source-id: af0e028d7fa7832a945aaee4e241ceb5418f4ec8
