wkdepends should inherit of the flags that have already been set.

In a project setup, with multiple dependencies, it is often the case that a define or a simple flags indicate how we want to build the project.

Therefore the user can assume that the flags setup in a parent project will be inherited in a child project( the original cmake behavior).

However this doesn't work at the moment in wkcmake, checked on linux with cmake 3.0.0.
