I don't think that's necessary, per above. You just have to access the vars via pointer indirection always, so long as *any* Pg version you support has ever lacked dllexport or DEF entry, so you can't dllimport the var.
You could enable direct dllimport if PG_VERSION_NUM shows you're on a new enough version, but you'd need to use conditionally compiled inlines or something to switch between the methods of accessing it, so there's not much point. You just declare
extern int* log_min_messages_p;
... and use that, probably also #define'ing log_min_messages away after including the Pg headers so that you can't reference it accidentally.
Actually, if __declspec(dllexport) or a .DEF entry was added in, say, 9.4.5, you could probably just:
#if PG_VERSION_NUM < 90405
extern int* log_min_messages_p;
#define log_min_messages (*log_min_messages_p)
#endif
after including all PostgreSQL headers. It won't work for inline functions defined in PostgreSQL headers, but should otherwise be OK I think.
