Mark all GUC variable as PGDLLIMPORT

Hi,

I've been pinged many time over the years to either fix Windows compatibility
or provide DLL for multiple extensions I'm maintaining. I've finally taken
some time to setup a Windows build environment so I could take care of most of
the problem, but not all (at least not in a satisfactory way).

I've also been looking a bit around other extensions and I see that the #1
problem with compiling extensions on Windows is the lack of PGDLLIMPORT
annotations, which is 99% of the time for a GUC.

This topic has been raised multiple time over the years, and I don't see any
objection to add such an annotation at least for all GUC variables (either the
direct variables or the indirect variables set during the hook execution), so
PFA a patch that takes care of all the GUC.

I don't now if that's still an option at that point, but backporting to at
least pg14 if that patch is accepted would be quite helpful.