fpurge now available in cygwin

[Eric Blake]

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Cygwin 1.7 now provides fpurge, and it passes test-fpurge.c without the
use of a wrapper function.  Therefore, I think that it avoids the BSD bug
reported here:
http://lists.gnu.org/archive/html/bug-gnulib/2007-04/msg00277.html

That means that blindly compiling the replacement fpurge.o is now a waste
on Cygwin (it is still necessary for BSD, where fpurge is broken; for
glibc, where it is spelled __fpurge and lacks a return value; and for
systems that lack a native fpurge).  But I'm struggling to come up with a
minimal test that can be used in fpurge.m4 to detect the difference
between the working Cygwin and the broken BSD fpurge implementations; the
referenced email doesn't show how to expose the BSD failure, just how to
work around it.  And it also seems that the comment is wrong:

         "The following always hold: if _flags & __SRD, _w is 0."
       If this invariant is not fulfilled and the stream is read-write but
       currently writing, subsequent putc or fputc calls will write directly
       into the buffer, although they shouldn't be allowed to.  */

But __SRD implies reading, not writing, so it seems like we need
s/currently writing/not &/.

- --
Don't work too hard, make some time for fun as well!

Eric Blake             address@hidden
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