Give shared_ptr the same treatment.

Here I have defined my own test for the feature. It's not an extensive test
but it checks for the presence of the class.

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

@@ -0,0 +1,7 @@
+#include <memory>
+
+int main()
+{
+	std::shared_ptr<int> test;
+	return 0;
+}