mingw: Use ftruncate() instead of _chsize_s()
mingw: Use ftruncate() instead of _chsize_s()

MinGW.org doesn't have _chsize_s() declared (in `io.h`), but it
(and Mingw-w64) does have `unistd.h`, with ftruncate() declared. So,
we make using _chsize_s(), and `io.h`, VC++ specific.

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