Use modern-style Eigen3 CMake variables

Change-Id: I4c14aa54552b3f718dff90eb397cb030aeb6ecd4

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 055bd0e..4851be5 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -239,8 +239,8 @@
 # Eigen.
 # Eigen delivers Eigen3Config.cmake since v3.3.3
 find_package(Eigen3 3.3 REQUIRED)
-if (EIGEN3_FOUND)
-  message("-- Found Eigen version ${Eigen3_VERSION}: ${EIGEN3_INCLUDE_DIRS}")
+if (Eigen3_FOUND)
+  message("-- Found Eigen version ${Eigen3_VERSION}: ${Eigen3_DIR}")
   if (CMAKE_SYSTEM_PROCESSOR MATCHES "^(aarch64.*|AARCH64.*)" AND
       Eigen3_VERSION VERSION_LESS 3.3.4)
     # As per issue #289: https://github.com/ceres-solver/ceres-solver/issues/289
@@ -258,7 +258,7 @@
     message("   which can still use the EIGEN_SPARSE_QR algorithm.")
     add_definitions(-DEIGEN_MPL2_ONLY)
   endif (EIGENSPARSE)
-endif (EIGEN3_FOUND)
+endif (Eigen3_FOUND)
 
 if (LAPACK)
   find_package(LAPACK QUIET)

cmake/CeresConfig.cmake.in

diff --git a/cmake/CeresConfig.cmake.in b/cmake/CeresConfig.cmake.in
index e5e2976..907646f 100644
--- a/cmake/CeresConfig.cmake.in
+++ b/cmake/CeresConfig.cmake.in
@@ -187,30 +187,30 @@
 
 # Eigen.
 # Flag set during configuration and build of Ceres.
-set(CERES_EIGEN_VERSION @EIGEN3_VERSION_STRING@)
+set(CERES_EIGEN_VERSION @Eigen3_VERSION@)
 # Search quietly to control the timing of the error message if not found. The
 # search should be for an exact match, but for usability reasons do a soft
 # match and reject with an explanation below.
 find_package(Eigen3 ${CERES_EIGEN_VERSION} QUIET)
-if (EIGEN3_FOUND)
-  if (NOT EIGEN3_VERSION_STRING VERSION_EQUAL CERES_EIGEN_VERSION)
+if (Eigen3_FOUND)
+  if (NOT Eigen3_VERSION VERSION_EQUAL CERES_EIGEN_VERSION)
     # CMake's VERSION check in FIND_PACKAGE() will accept any version >= the
     # specified version. However, only version = is supported. Improve
     # usability by explaining why we don't accept non-exact version matching.
     ceres_report_not_found("Found Eigen dependency, but the version of Eigen "
-      "found (${EIGEN3_VERSION_STRING}) does not exactly match the version of Eigen "
+      "found (${Eigen3_VERSION}) does not exactly match the version of Eigen "
       "Ceres was compiled with (${CERES_EIGEN_VERSION}). This can cause subtle "
       "bugs by triggering violations of the One Definition Rule. See the "
       "Wikipedia article http://en.wikipedia.org/wiki/One_Definition_Rule "
       "for more details")
   endif ()
   ceres_message(STATUS "Found required Ceres dependency: "
-    "Eigen version ${CERES_EIGEN_VERSION} in ${EIGEN3_INCLUDE_DIRS}")
-else (EIGEN3_FOUND)
+    "Eigen version ${CERES_EIGEN_VERSION} in ${Eigen3_DIR}")
+else (Eigen3_FOUND)
   ceres_report_not_found("Missing required Ceres "
     "dependency: Eigen version ${CERES_EIGEN_VERSION}, please set "
     "Eigen3_DIR.")
-endif (EIGEN3_FOUND)
+endif (Eigen3_FOUND)
 
 # glog (and maybe gflags).
 #