Port the config discovery script to pkg-config

[anmonteiro]

• PostgreSQL installations also add pkg-config configuration files.
• In cross-compilation scenarios, calling out to pg_config will list the wrong --libdir
• This change uses pkg-config to find the C include & library flags for libpq
  • I've kept a fallback to the current pg_config-based implementation to avoid errors in cases that users don't have pkg-config in their $PATH.

also slightly related to ocaml/dune#1492 (comment)