So I had a quick look at the code and found the lines in need of a fix.  The problem comes as usual from our friend “path + optimize and 'o' or 'c'”.  I can read some C, so I tried to replace that with imp.cache_from_source, but a real C programmer should take it from here, especially if they find themselves on Windows so that they can recompile the wininst*.exe file and test the fix :)