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-class_override_final.cpp

@@ -0,0 +1,21 @@
+class base {
+public:
+    virtual int foo(int a)
+     { return 4 + a; }
+    int bar(int a) final
+     { return a - 2; }
+};
+
+class sub final : public base {
+public:
+    virtual int foo(int a) override
+     { return 8 + 2 * a; };
+};
+
+int main(void)
+{
+    base b;
+    sub s;
+
+    return (b.foo(2) * 2 == s.foo(2)) ? 0 : 1;
+}