Set TORCH_CXX_FLAGS in minimal example (#29890)
Summary:
To avoid ABI issue
EDIT: After this PR, the example CMakeLists.txt will always use the `-D_GLIBCXX_USE_CXX11_ABI` value set in `share/cmake/Torch/TorchConfig.cmake`, regardless of the `-D_GLIBCXX_USE_CXX11_ABI` value passed to the `cmake` command by the user.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/29890
Differential Revision: D18531391
Pulled By: yf225
fbshipit-source-id: 2db78ae7a33a4088b579e81c60b9a74861f1ccde
