[issue21107] Add pgen.vcxproj to allow regenerating grammar files on Windows

[Zachary Ware]

Zachary Ware added the comment:

> I don't even know what pgen is or why I'd want to build it...

pgen is the parser generator, taking Grammar/Grammar as input and generating Include/graminit.h and Python/graminit.c.  Only useful when you're changing Python's grammar, but without the ability to build pgen, it's hard for Windows developers to play with the grammar at all.

> If running it updates a file that would trigger a rebuild, then I'd
> really like it to copy what _freeze_importlib currently does as far as
> writing to a temporary file and conditionally overwriting the actual
> one. That is the most reliable way to avoid unnecessary rebuilds.

_freeze_importlib was what I used for a guide initially, and will be again when I get around to redoing the patch :)

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<http://bugs.python.org/issue21107>
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