Add debug support for MSVC builds

[jayvdb]

I dont see --debug support in the Windows build scripts. Debug builds are supported by the CPython PCbuild system. https://github.com/python/cpython/blob/master/PCbuild/readme.txt

[indygreg]

Yes, this would be nice. The hacks to the Windows build system are ugly and the lack of debug support is mainly a side-effect of me wanting to preserve more of my sanity by not having to hack on the Visual Studio builds any more than necessary.