Use exported symbols list on macOS for loadable modules as well

Use exported symbols list on macOS for loadable modules as well

On macOS, when building with the make system, the exported symbols
list $(SHLIB_EXPORTS) was ignored.  This was probably not intentional,
it was probably just forgotten, since that combination has never
actually been used until now (for libpq-oauth).

The meson build system handles this correctly.  Also, other platforms
have been doing this correctly.

This fixes it.  It also does a bit of refactoring to make the code
match the layout for other platforms.

Reviewed-by: Jacob Champion <jacob.champion@enterprisedb.com>
Discussion: https://www.postgresql.org/message-id/flat/c70ca32e-b109-460d-9810-6e23ebb4473f%40eisentraut.org