Fix 'make install' broken by faulty FS_X install dependency.

* configure.ac
  (SVN_LIBSVN_FS_LINKS_FS_X): remove bogus dependency;
                              add comment about the correct one

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1510427 13f79535-47bb-0310-9956-ffa450edef68