The variant.props may have incompatible build options which break
the mpy-cross build and in any case mpy-cross has nothing to do
with variant support.
This is in line with the change made for other ports in d53c3b6a: since
the default output directory already includes the variant name in it
there's no need to add it to the executable as well.
Append to PyIncDirs, used to define include directories specific to
MicroPython, instead of just overwriting it so project files importing this
file can define additional directories. And allow defining the target
directory for the executable instead of hardcoding it to the windows
directory. Main reason for this change is that it will allow building
mpy-cross with msvc.
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.
