[shared_ptr] Introduced an error message that explains why the test fails for gcc <4.6

[[Imported from SVN: r7083]]

dune/common/test/shared_ptrtest.cc

@@ -157,7 +157,13 @@ int main(){
     assert(bar);
 
     // test constructor from nullptr
+#if defined(SHARED_PTR_HEADER) && !defined(HAVE_NULLPTR)
+        #error Construction of shared_ptr from a nullptr will not work as compiler
+        #error does not support the latter.
+    shared_ptr<double> bar_null=shared_ptr<double>();
+#else
     shared_ptr<double> bar_null(nullptr);
+#endif
     assert(!bar_null);
     assert(!bar_null.get());