Fix missing EIGEN_VERSION expansion typo.

Change-Id: I1f481f5cb618f1c4dfe48e2bf4607f7801f65991

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 1565413..1196521 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -265,7 +265,7 @@
   IF (EIGEN_VERSION VERSION_LESS 3.1.0)
     MESSAGE(FATAL_ERROR "-- Ceres requires Eigen version >= 3.1.0 in order "
       "that Eigen/SparseCore be available, detected version of Eigen is: "
-      "$EIGEN_VERSION")
+      "${EIGEN_VERSION}")
   ENDIF (EIGEN_VERSION VERSION_LESS 3.1.0)
 
   MESSAGE("-- Found Eigen version ${EIGEN_VERSION}: ${EIGEN_INCLUDE_DIRS}")