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.cmake

@@ -130,3 +130,4 @@ cxx11_check_feature("rvalue-references" HAS_CXX11_RVALUE_REFERENCES)
 cxx11_check_feature("sizeof_member" HAS_CXX11_SIZEOF_MEMBER)
 cxx11_check_feature("static_assert" HAS_CXX11_STATIC_ASSERT)
 cxx11_check_feature("variadic_templates" HAS_CXX11_VARIADIC_TEMPLATES)
+cxx11_check_feature("shared_ptr" HAS_CXX11_SHARED_PTR)

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

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