Cannot use external Eigen library with Mitk 2016-03

Summary:
A CMake variable export is missing in SuperBuild.cmake, which prevent Mitk to compile with the use of a external (ie system) Eigen library.
T22603: Cannot use external Eigen library with Mitk 2016-03 (patch provided)

Test Plan: Manual testing.

Reviewers: goch

Reviewed By: goch

Subscribers: aurelien

Tags: MITK

Maniphest Tasks: T22603

Differential Revision: https://phabricator.mitk.org/D38