Make WkIncludedir accept multiple directories

[asmodehn]

It has been observed that it s very useful to be able to specify multiple include directories for real world projects.
Also this variable receive source dependencies header directories.

So we should make it possible to declare multiple include folders.
However this can have broad implications and we should be careful when implementing that change.

[asmodehn]

WkCocos and all it s subproject hierarchy is a good project to test this feature.