But if the implementation is in a source-file and not in a header, you will need the sources so the CodeCompletion-plugin (which offers this functionality) can parse them.
And they must be either part of your project or in a project which is also opened in the same workspace as your project.
In this case you need to switch on the option to parse the whole workspace (as far as I know).
But find declaration should work in any cases,because to use the lib, you also need the include headers anyway.
Note, this will probably not work (correctly) with templates and if you use something like pkg-config or wx-config.