Supplied Variables

[larryk85]

In conjunction with #73 we should also have some generated variables.

For each object (i.e. app, lib) we should produce a variable of the object name and a set of attributes with that variable.

Current attributes needed are:

1. Object's source directory : src_dir
2. Object's include directory : include_dir
3. Object's compile flags : cflags
4. Object's link flags : ldflags

These would be used something like this:

In the project.yaml ... we have the app foo and the lib bar

compile_flags : "{foo.cflags} {bar.include_dir}/../other"