return_value_ptr and return_value_used are removed in PHP 7.

We don't use return_value_ptr (other then to cast it to (void) to avoid unused-variable warnign), but return_value_used to do use in couple of cases to skip bson encoding/decoding when we know it serves no purpose.

The return_value_used should be hardcoded to 1 in PHP 7 as there is nothing similar to it anymore.

For the unused-argument warning, we can create a version macro for it.