macos exported symbols list not used for loadable modules

On macOS, when building with the make system, the exported symbols list
(SHLIB_EXPORTS) is ignored. I don't think that is intentional. It was
probably just forgotten, since that combination has never actually been
used until now (for libpq-oauth).

(Alternatively: Am I missing something? Is this combination not useful?
Was it not supported in older versions at some point? I couldn't
think of anything.)

The meson build system handles this correctly.

I suggest that we fix this. I attach patch 0001 with a minimal fix,
patch 0002 is a bit more refactoring to make the code match the layout
for other platforms.