Make it possible to generate constexpr signatures in C++11 mode

The current C++14 constexpr signatures don't require relaxed constexpr,
but only `auto` return type deduction. To get around this in C++11,
the type caster's `name()` static member functions are turned into
`static constexpr auto` variables.

include/pybind11/functional.h

@@ -75,10 +75,8 @@ public:
             return cpp_function(std::forward<Func>(f_), policy).release();
     }
 
-    PYBIND11_TYPE_CASTER(type, _("Callable[[") +
-            argument_loader<Args...>::arg_names() + _("], ") +
-            make_caster<retval_type>::name() +
-            _("]"));
+    PYBIND11_TYPE_CASTER(type, _("Callable[[") + argument_loader<Args...>::arg_names + _("], ")
+                               + make_caster<retval_type>::name + _("]"));
 };
 
 NAMESPACE_END(detail)