Make enable_if more portable

As I learned clang is known to not work correctly
if enable_if is used in a varadic template parameter
of type 'class'. Replacing this by e.g. an int-parameter
should fix the problem.

dune/solvers/solvers/criterion.hh

@@ -51,7 +51,7 @@ namespace Dune {
        * of the header string coincide to get a readable log.
        */
       template<class F,
-        enable_if_t<std::is_convertible<result_of_t<F()>, Result>::value>...>
+        enable_if_t<std::is_convertible<result_of_t<F()>, Result>::value, int> = 0>
       Criterion(F&& f, const std::string& header) :
         f_(std::forward<F>(f)),
         header_(header)
@@ -75,7 +75,7 @@ namespace Dune {
        * of the header string coincide to get a readable log.
        */
       template<class F,
-        enable_if_t<std::is_convertible<result_of_t<F()>, std::string>::value>...>
+        enable_if_t<std::is_convertible<result_of_t<F()>, std::string>::value, int> = 0>
       Criterion(F&& f, const std::string& header) :
         f_( [=]() {return std::make_tuple(false, f());} ),
         header_(header)