This turns out not to be as harmless as I thought: MSVC will complain
if it sees an "extern" declaration without PGDLLEXPORT and then one with.
(Seems fairly silly, given that this can be changed after the fact by the
linker, but there you have it.)  Therefore, contrib modules that have
extern's for V1 functions in header files are falling over in the
buildfarm, since none of those externs are marked PGDLLEXPORT.

We might or might not conclude that we're willing to plaster those
declarations with PGDLLEXPORT in HEAD, but in any case there's no way we're
going to ship this change in the back branches.  Third-party authors would
not thank us for breaking their code in a minor release.  Hence, revert
the addition of PGDLLEXPORT (but let's keep the extra info in the comment).
If we do the other changes we can revert this commit in HEAD.

Per buildfarm.