Note this still needs some work: currently all source files are always
preprocessed no matter which one actually changed, moreover that happens
file by file without any parallellism so builds are painstakingly slow.

This fixes the build after adding directory iteration in d874702f

This allows multiple versions (e.g. Debug/Release, x86/x64) of micropython.exe
to co-exist instead and also solves potential problems where msbuild does not
completely rebuild the output and/or pdb files when switching between builds,
which in turn can cause linker errors in dependent projects.

By default exe/map/... files go in windows/build/$(Configuration)$(Platform)

After each build micropython.exe is still copied from the above directory to
the windows directory though, as that is consistent with the other ports and
the test runner by default uses that location as well.

Also rename env.props -> path.props which is a clearer name,
and add ample documentation in the affected build files.

(also see discussion in #1538)

This defaults to 'python' but can be now overridden if needed

Fixes issues like #1532

- add SEEK_XXX definitions, this fixes missing definition in py/stream.c
- move R_OK from realpath.c and add W_OK/F_OK defintions
- move STDXXX_FILENO definitions from mpconfigport for consistency

This allows using (almost) the same code for printing floats everywhere,
removes the dependency on sprintf and uses just snprintf and
applies an msvc-specific fix for snprintf in a single place so
nan/inf are now printed correctly.

Also rename py-version.h to mpversion.h for consistency with mpconfig.h.

Some files are excluded, otherwise a whole lot of files need converting.

Remove some duplication in the code for generating
qstrdefs.generated.h and py-version.h

- 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.

The pointers to the bss section are acquired in init.c()
by inspecting the PE header. Works for msvc and mingw.

This fixes generating the headers casuing complete rebuilds,
even when the headere's content didn't really change.

Extend the windows port so it compiles with the toolchain from Visual Studio 2013