[Python] Default PYARROW_CMAKE_GENERATOR can yield broken libraries with MSVC if the C++ toolchain is different

See discussion in https://github.com/apache/arrow/pull/6350.

Python's setup.py defaults to the "Visual Studio 14 2015 Win64" CMake generator here

https://github.com/apache/arrow/blob/apache-arrow-0.16.0/python/setup.py#L130

We found in ARROW-6757 that if VS 2017 or newer was used to build the C++ libraries, then there can be a toolchain conflict causing segfaults. I'm not sure if there's a better way to infer which VS toolchain is "preferred" (based on what "VsDevCmd.bat" was run), but we should see if we should do something other than what we have now