C++ shared library is consumed by a C++ project in the same composite build

@adammurdoch commented on [Mon May 22 2017](https://github.com/gradle/stable-native/issues/20)

Build on #19 to allow a C++ library to be consumed from another build in the same composite build.

- Allow a C++ project to declare a dependency on an external library.
- Attach the appropriate meta-data to the consuming and producing projects to allow this to work in a composite build.

Note that there is no publishing support in this story. The "external" library can only be used in a composite build, when built from source locally.

Start by reading issue #19 and the composite-build behavior described here, then trace how C++ projects currently declare and resolve dependencies. Define the consuming and producing project metadata needed for source-built external libraries, and verify that publishing remains unsupported while the dependency works in a composite build.

Written by the indexing model from the issue text.