Fix compilation issues with eigen 5

[ipadjen]

Eigen recently published eigen5 and the current CMakeLists ends up in an error:

CMake Error at CMakeLists.txt:81 (target_link_libraries):
  Target "val3dity" links to:

    CGAL::Eigen3_support

  but the target was not found.  Possible reasons include:

    * There is a typo in the target name.
    * A find_package call is missing for an IMPORTED target.
    * An ALIAS target is missing.

This small change should fix it