set some function template parameters explicitly because Clang 3.2 canno...

...t infer them from the arguments.

The corresponding signature of make_var is:

template < class R, class O, class O1 >
variable<R>
make_var(O1 (O::*mem_func)() const, R (O1::object_type::*mem_func_1)() const);

I am not sure why GCC can deduce O1 but Clang cannot. My commit works for both Clang 3.2 and GCC 4.8.