* m4/acl.m4 (gl_FUNC_ACL): Update comments regarding Cygwin.
* lib/acl-internal.h: Likewise.
(HAVE_ACL_EXTENDED_FILE): Undefine on Cygwin.
* lib/acl-internal.c: Update comments regarding Cygwin.
* lib/acl_entries.c: Likewise.
* lib/file-has-acl.c: Likewise.
(file_has_acl): For Cygwin, use a different way to determine whether
the "default" ACL of a directory is nontrivial.
* lib/get-permissions.c: Update comments regarding Cygwin.
* lib/set-permissions.c: Likewise.