fts finds no files under mingw

[kodis]

I've found a problem with the gnulib fts module.  When using the mingw
cross compiler, the fts_read routine never finds any files or
directories other than the directories supplied to the preceeding
fts_open call.  It seems that this will happen on any platform that
doesn't have a working dirfd function available.

I've traced the problem down to the dirfd call around line 1160 in
fts.c, in the internal fts_build routine, after the comment block
containing the admonition that "This is all fairly nasty."

After getting a valid DIR *dirp, the line "int dir_fd = dirfd(dirp);"
gets a dir_fd value of -1, but with an errno value of 0.  This happens
because the dirfd.m4 macro recognizes that there's no way to get the file
descriptor associated with an open DIR* on this platform, and so
#defines the return value of dirfd to always be -1.

This in turn results in dirp being set to NULL, which prevents the
following readdir loop from being run to generate the list of files in
the directory.

-- John Kodis.