Add C++11 compiler support detection feature

This is currently copied from http://quickgit.kde.org/index.php?p=scratch%2Fdakon%2Fcmake-cxx11.git
but there is a chance that in future it will be merged into CMake proper.

cmake/Modules/CheckCXX11Features/cxx11-test-variadic_templates.cpp

@@ -0,0 +1,23 @@
+int Accumulate()
+{
+	return 0;
+}
+
+template<typename T, typename... Ts>
+int Accumulate(T v, Ts... vs)
+{
+	return v + Accumulate(vs...);
+}
+
+template<int... Is>
+int CountElements()
+{
+	return sizeof...(Is);
+}
+
+int main()
+{
+	int acc = Accumulate(1, 2, 3, 4, -5);
+	int count = CountElements<1,2,3,4,5>();
+	return ((acc == 5) && (count == 5)) ? 0 : 1;
+}