Build seems to fail with PostgreSQL 12 version, due to incompatibility
after [1], and required presence of libpgport (to expose pg_sprintf for
scram implementation).
To improve the situation, introduce couple of changes:
* Rely on pg_config to get lib/include directories
* Add libpgport.a into linking
* Provide different versions of pg_b64_decode/encode depending on
PG_VERSION_NUM
[1]: https://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=cfc40d384ae51ea2886d599d2008ae57b529e6ea
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