Adding libpgcommon and libpgcore to libpq pkg-config's Requires.private

Hi,

I recently investigated a build with statically linked libpq
that broke on update to PostgreSQL 12.0.

The problem was that starting from 12.0, libpq doesn't
include some required files via symlinks anymore,
and needs to be linked against libpgcommon and
libpgcore. (When dynamically linking, this problem
doesn't manifest.)

(Ref: my earlier mail.
https://www.postgresql.org/message-id/CACS8yHL0gbL3ECONrDygcBioTQBY%3DoVG-KGWB6%2BN7spG%2BeSMQw%40mail.gmail.com)

However, the pkg-config file for libpq doesn't reflect that fact.
I think that this should be considered as a bug; I think that
libpq's pkg-config file should be have libpgcommon and libpgcore
on the Libs.private row like this:

Libs.private: -L/musl/lib/ -lm -lpgcommon -lpgport

That would allow scripts that get their linking flags from
pkg-config to correctly build libpq even when doing a static build.

Let me hear what you think.

All the best,
Pyry Kontio