libpq-oauth: Never link against libpq's encoding functions

libpq-oauth: Never link against libpq's encoding functions

Now that libpq-oauth doesn't have to match the major version of libpq,
some things in pg_wchar.h are technically unsafe for us to use. (See
b6c7cfac8 for a fuller discussion.) This is unlikely to be a problem --
we only care about UTF-8 in the context of OAuth right now -- but if
anyone did introduce a way to hit it, it'd be extremely difficult to
debug or reproduce, and it'd be a potential security vulnerability to
boot.

Define USE_PRIVATE_ENCODING_FUNCS so that anyone who tries to add a
dependency on the exported APIs will simply fail to link the shared
module.

Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Reviewed-by: Zsolt Parragi <zsolt.parragi@percona.com>
Discussion: https://postgr.es/m/CAOYmi%2BmrGg%2Bn_X2MOLgeWcj3v_M00gR8uz_D7mM8z%3DdX1JYVbg%40mail.gmail.com