Add global to avoid unnecessary system calls on systems where umask does not affect symlinks

git-svn-id: http://svn.savannah.nongnu.org/svn/rdiff-backup@776 2b77aa54-bcbc-44c9-a7ec-4f6cf2b41109

trunk/rdiff-backup/CHANGELOG

 New in v1.1.8 (????/??/??)
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+Test for symlink permissions support to avoid unncessary syscalls on
+platforms that don't support them. (Patch from Andrew Ferguson.)
+
 RPM specfile update from Gordon Rowell.