Use LINK_PRIVATE to link exported libraries dependencies

Now that we depend on CMake 2.8.7 we can use it.

(partially cherry picked from commit 50c3c56ce85abe3653a3efa56590b0d915460a83)

iCub should have been fixed after I pushed the change the first time, it builds for me without changes, but I'm not building all the devices, etc, so I cannot ensure that there won't be problems. Anyway it is quite easy to fix.