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-constexpr.cpp

@@ -0,0 +1,19 @@
+constexpr int square(int x)
+{
+	return x*x;
+}
+
+constexpr int the_answer()
+{
+	return 42;
+}
+
+int main()
+{
+	int test_arr[square(3)];
+	bool ret = (
+		(square(the_answer()) == 1764) &&
+		(sizeof(test_arr)/sizeof(test_arr[0]) == 9)
+	);
+	return ret ? 0 : 1;
+}