libstdc++: Fix link failure in _OutputIteratorConcept

libstdc++: Fix link failure in _OutputIteratorConcept

The C++98-style concept check for output iterators causes a link
failure on mingw-w64, because the __val() member function isn't defined.
Change it to use a function pointer instead. That pointer is never set
to anything meaningful, but it doesn't matter as the __constraints()
function only has to be instantiated, it's never called.

We could refactor all of these to use unevaluated contexts (e.g. sizeof
of __decltype) so that we only check the expressions are well-formed,
without any codegen at all. Any improvements to these are very low
priority though.

libstdc++-v3/ChangeLog:

	* include/bits/boost_concept_check.h (_OutputIteratorConcept):
	Change member function to data member of function pointer type.