Clean up warnings in MinGW builds.

Experimentation with modern MinGW (specifically the 5.0.2 version packaged
for Fedora 26) shows that its version of sys/stat.h *does* provide S_IRGRP
and friends, contrary to the expectation of win32_port.h.  This results in
an astonishing number of compiler warnings, and perhaps in incorrect code
--- I'm not sure if the nonzero values supplied by MinGW's header actually
do anything.  Hence, adjust win32_port.h to only define these macros if
<sys/stat.h> doesn't.

This might be worth back-patching, but given the lack of complaints so
far, I'm not too excited about it.

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