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)
Merge pull request #41 from drdanz/LINK_PRIVATE

Use LINK_PRIVATE to link exported libraries dependencies
Clean some CMake stuff in the extern directory

* Link GTK2 libraries instead of exporting it with the XXX_LIBRARIES
  variable
* Remove XXX_LINK_DIRECTORIES as we always export the full path of the
  library
* Export missing directory with XXX_INCLUDE_DIRS
* Link libraries using LINK_PRIVATE
* Add XXX_DEFINITIONS variables

Should fix some problems with the experimental build on the dashboard
and with YARP_USE_GTK2=ON when the goocanvasmm library is not
installed on the system
Merge branch 'master' into yarp-2.4