Fall back to `pkg-config` if `pg_config` is not found

[schuetzm]

Apparently openSUSE no longer install pg_config with the client libraries, as they say it's only supposed to be used for building server extensions:

https://lists.opensuse.org/archives/list/factory@lists.opensuse.org/thread/QGFXOR7WM2HVX6MPHY76ADCFGM7ETEKA/

See also #366.