MinGW support

[Cyril Arnould]

Hi! I was wondering if there's a possibilty for GNU findutils to
officially start supporting MinGW-w64. It has been ported before as
part of both the GnuWin32 and ezwinports projects, those however are
based on by now rather dated versions of findutils which come from
before the introduction of the fts based find command.

Last year, I have worked on a port of the newest findutils release
based off the patches in the ezwinports project. As far as I'm aware,
the issues that the patches address are not covered by Gnulib as of
yet. The changes can still quite easily be rebased onto master in case
you want to take a look:

https://github.com/cyrilarnould/findutils

I'm quite happy with how it turned out in terms of functionality, but
some of the changes are quite hacky and probably need polishing before
they could be included. Me being very inexperienced in C programming
and the workflow around autotools didn't help. Some newer features I
just skipped because I did not know how I would implement them; they
were not around in the ezwinports version either.