No recommended practices for non-system libraries

[spease]

It would be nice if the manual provided an example of a non-system library. There are probably two common cases:

• The source code is available and the library needs to be checked out and perhaps built
• The library is prebuilt and distributed with libraries and header files

Specifically the point of ambiguity right now is what path I should put in link() for the generated code to work correctly on someone's system who's trying to build the Rust library. This will likely become clear eventually, but it'd be nice to know up-front while I'm writing the build script.