Setup the symlink to /sys to mirror one's current source, e.g. if my source
tree was /usr/src/svn, /sys would point to usr/src/svn

This fixes the assumption that the source tree will always exist at
${DESTDIR}/usr/src

MFC after: 1 week
PR: 76362
Reported by: Scot Hetzel <swhetzel@gmail.com>
Sponsored by: EMC / Isilon Storage Division