Linker fails if the name of the library passed to .compile is the same as an external static library

[Enduriel]

This was really difficult to debug for me as a complete newb to external rust bindings, though in hindsight it makes perfect sense. I would adjust part documentation for the Build::compile function from
must be unique across all compile invocations made by the same build script.
to
must be unique across all compile invocations made by the same build script and the names of any statically linked libraries

I wanted to make a PR for this but I have no idea where the documentation is actually written since a ctrl+f on 'must be unique' in the cxx repo returns nothing.