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