Really include "_cv_" in all instances of dune_boost_fusion. (Oops)

[[Imported from SVN: r6159]]

m4/boost_fusion.m4

@@ -19,7 +19,7 @@ AC_DEFUN([DUNE_BOOST_FUSION],
                                     boost::fusion::vector<int,char,double> v;
                                     return 0;
                                    ]]),
-                               dune_boost_fusion=yes, dune_boost_fusion=no)
+                               dune_cv_boost_fusion=yes, dune_cv_boost_fusion=no)
                                AC_LANG_POP([C++])])
          if test "x$dune_boost_filesystem" = "xyes"; then
                AC_DEFINE(HAVE_BOOST_FUSION,,[define if the Boost::Fusion headers are available])