When building extensions, honor options set in CMake. (#21653)
Summary:
Currently when building extensions, variables such as USE_CUDA, USE_CUDNN are used to determine what libraries should be linked. But we should use what CMake has detected, because:
1. If CMake found them unavailable but the variables say some libraries should be linked, the build would fail.
2. If the first build is made using a set of non-default build options, rebuild must have these option passed to setup.py again, otherwise the extension build process is inconsistent with CMake. For example,
```bash
# First build
USE_CUDA=0 python setup.py install
# Subsequent builds like this would fail, unless "build/" is deleted
python setup.py install
```
This commit addresses the above issues by using variables from CMakeCache.txt when building the extensions.
---
The changes in `setup.py` may look lengthy, but the biggest changed block is mostly moving them into a function `configure_extension_build` (along with some variable names changed to `cmake_cache_vars['variable name']` and other minor changes), because it must be called after CMake has been called (and thus the options used and system environment detected by CMake become available).
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21653
Differential Revision: D15824506
Pulled By: ezyang
fbshipit-source-id: 1e1eb7eec7debba30738f65472ccad966ee74028
