NOTE I'm not a developer, but I just happen to search for the issue some
more and happened to find a similar issue.
In the issue, the problem is "libpq", not "libpq" --- but I do know that
it was probably a typo ---, furthermore, there is a similar issue that
should provide us with some hints:
[[https://github.com/pgmodeler/pgmodeler/issues/43] ].
However, that issue doesn't seem to indicate if the suggested .pc file
was ever added, so perhaps we must ask upstream to do so now.