msvc: Define main build/include directories in a single location
- Use a single file env.props for defining the main directories used when building. env.props resolves the base directory and defines overridable output directories, and is used by all other build files. - Fix the build currently failing, basically because the preprocessing command for generating qstrdefs uses different include directories than the build itself does. (specifically, qstrdefs.h uses #include "py/mpconfig.h" since the fixes for #1022 in 51dfcb4b, so we need to use the base directory as include directory, not the py dir itself). So define a single variable containing the include directories instead and use it where needed.