[docs] [issue17386] Bring Doc/make.bat as close to Doc/Makefile as possible

[Zachary Ware]

Zachary Ware added the comment:

I caught a small oversight.  This new patch changes the example in README.txt to ``make html "PYTHON2=C:\Python27\python.exe"`` (PYTHON->PYTHON2).  Also, I added a bit of backward compatibility to make.bat; %PYTHON2% will default to %PYTHON% if it is set.  Otherwise, %PYTHON% is ignored and unchanged.

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Added file: http://bugs.python.org/file29876/win_doc_make.v4.diff

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<http://bugs.python.org/issue17386>
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