Fix config_file compilation errors with MSVC

Previous fix 4ae33c for ticket #6797 introduced a new problem with
Visual Studio compilers about two missing methods in
'common_config_file_iterator'. Adding those seemingly not required
methods as empty methods fixes the issue. As an added bonus, the
bogus warnings about DLL-interfaces get a silencing treatment, too.

Tests were run with vc10, vc11, and vc12. Without this fix, the
affected test cases fail on all 3 compilers. With the fix in place,
all tests pass.

Signed-off-by: Daniela Engert <dani@ngrt.de>