Fix CGAL support for CGAL version 6+ (#56)

CGAL defines proper CMake targets since v5, and the previous way of including CGAL from CMake was removed with v6.

CMakeLists.txt

@@ -171,8 +171,12 @@ if (KAGEN_USE_CGAL)
     set(CGAL_DONT_OVERRIDE_CMAKE_FLAGS TRUE CACHE BOOL "Force CGAL to maintain CMAKE flags")
     find_package(CGAL QUIET)
     if (CGAL_FOUND)
-        add_definitions(-DKAGEN_CGAL_FOUND) 
+      add_definitions(-DKAGEN_CGAL_FOUND)
+      if (${CGAL_VERSION} VERSION_GREATER_EQUAL 5)
+	list(APPEND KAGEN_LINK_LIBRARIES CGAL::CGAL)
+      else()
         include(${CGAL_USE_FILE})
+      endif()
     else ()
         message(STATUS "Could not find the CGAL library: Random Delaunay Graphs will not be available")
     endif ()