Add minimal shared_ptr support

Enable the tests and support of shared_ptr in them for C (which required
disabling a previously passing, because not doing anything, attributes
test which is currently broken for unrelated reasons).

Examples/test-suite/c/cpp11_shared_ptr_const_runme.c

@@ -0,0 +1,16 @@
+#include "cpp11_shared_ptr_const_wrap.h"
+#include <assert.h>
+
+int main(int argc, const char *argv[]) {
+    Foo* f;
+    Foo* f2;
+
+    f = Foo_new(17);
+    assert(Foo_get_m(f) == 17);
+    f2 = cpp11_shared_ptr_const_foo(f);
+    assert(Foo_get_m(f2) == 17);
+    Foo_delete(f2);
+    Foo_delete(f);
+
+    return 0;
+}