We want the .vcxproj to be just a container with the minimum content for
making it work as a project file for Visual Studio and MSBuild, whereas the
actual build options and actions get placed in separate reusable files.
This was roughly the case already except some compiler options were
overlooked; fix this here: we'll need those common options when adding a
project file for building mpy-cross.
This is to keep the top-level directory clean, to make it clear what is
core and what is a port, and to allow the repository to grow with new ports
in a sustainable way.
