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 <[email protected]>

Author: DanielaE

include/boost/program_options/detail/config_file.hpp

@@ -27,6 +27,11 @@
 #include <boost/type_traits/is_same.hpp>
 #include <boost/shared_ptr.hpp>
 
+#ifdef BOOST_MSVC
+# pragma warning(push)
+# pragma warning(disable: 4251) // class XYZ needs to have dll-interface to be used by clients of class XYZ
+#endif
+
 
 
 namespace boost { namespace program_options { namespace detail {
@@ -77,6 +82,11 @@ namespace boost { namespace program_options { namespace detail {
 
         void get();
 
+#if BOOST_WORKAROUND(_MSC_VER, <= 1800)
+        void decrement() {}
+        void advance(difference_type) {}
+#endif
+
     protected: // Stubs for derived classes
 
         // Obtains next line from the config file
@@ -177,4 +187,8 @@ namespace boost { namespace program_options { namespace detail {
 
 }}}
 
+#ifdef BOOST_MSVC
+# pragma warning(pop)
+#endif
+
 #endif