FoundationDB-Client package should install pkgconfig file

One of the users of swift bindings reported that they couldn't use FoundationDB binding without manually installing the pkgconfig file (https://github.com/FoundationDB/fdb-swift-bindings/pull/20/files).

Looked at my local installation and I didn't see it either. I see we have some *.pc file in the repo, so probably should include that as part of the client package we release.

Start by locating the existing *.pc files and the configuration for the FoundationDB client package. Compare the released package contents with what the Swift bindings expect in fdb-swift-bindings pull request #20. Done means installing the client package makes its pkgconfig file available without a manual copy.

Written by the indexing model from the issue text.