Activating test for variadic templates, as debugallocator uses

variadic templates if available. 

This breaks building of the test of debug allocator as it uses std::forward.

[[Imported from SVN: r7453]]

m4/dune_common.m4

@@ -22,6 +22,7 @@ AC_DEFUN([DUNE_COMMON_CHECKS],
   AC_REQUIRE([STATIC_ASSERT_CHECK])
   AC_REQUIRE([NULLPTR_CHECK])
   AC_REQUIRE([SHARED_PTR])
+  AC_REQUIRE([VARIADIC_TEMPLATES_CHECK])
   AC_REQUIRE([DUNE_BOOST_BASE])
   AC_REQUIRE([MAKE_SHARED])
   AC_REQUIRE([DUNE_LINKCXX])