FoundationDB-Client package should install pkgconfig file

[vishesh]

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.

[xis19]

Maybe also a .cmake file