Re: problem with msvc linker - cannot build orafce

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.