FindEigen3: Set Eigen3_FOUND variable

cmake/FindEigen3.cmake

diff --git a/cmake/FindEigen3.cmake b/cmake/FindEigen3.cmake
index 52efb4e..4d9bc1a 100644
--- a/cmake/FindEigen3.cmake
+++ b/cmake/FindEigen3.cmake
@@ -68,6 +68,7 @@
   # in cache already
   _eigen3_check_version()
   set(EIGEN3_FOUND ${EIGEN3_VERSION_OK})
+  set(Eigen3_FOUND ${EIGEN3_VERSION_OK})
 
 else (EIGEN3_INCLUDE_DIR)