pgsql: 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(dot)evan(dot)chao(at)gmail(dot)com>
Reviewed-by: Zsolt Parragi <zsolt(dot)parragi(at)percona(dot)com>
Discussion: https://postgr.es/m/CAOYmi%2BmrGg%2Bn_X2MOLgeWcj3v_M00gR8uz_D7mM8z%3DdX1JYVbg%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/dba35604485feff679893f22a79e8c0f3ee6fc0e

Modified Files
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src/interfaces/libpq-oauth/Makefile | 11 +++++++++--
src/interfaces/libpq-oauth/meson.build | 10 +++++++++-
2 files changed, 18 insertions(+), 3 deletions(-)