Log Message:
Copy gnulib readlinkat patch from archivers/gtar-base.

This undefines readlink so that the twisty maze of symlinks builds,
and fixes the build on netbsd-5.  Confirmed to work on netbsd-5/amd64
and netbsd-6/i386.  Thanks to obache@ for pointing out the problem.

I will request a pullup to 2014Q4 soon if there are no objections.