[CMake] Passing requirements to users

[James Mansion]

>There will probably be a way to set the *interface*
>include file search path separately from the
>*implementation* search path.  Then when a library is
>linked its interface include file search path can be 
>included during compilation of source files using the
>library.

Great - and hopefully not just a -I setting but also
arbitrary -D settings etc as well (possibly varying
according to the way the library was built).

James