Eigen version moved in 5.x.x, find version for both 3.x.x and 5.x.x

Author: EricaCMitchell

cmake/modules/FindEigen3.cmake

@@ -31,7 +31,18 @@ if(NOT Eigen3_FIND_VERSION)
 endif(NOT Eigen3_FIND_VERSION)
 
 macro(_eigen3_check_version)
-  file(READ "${EIGEN3_INCLUDE_DIR}/Eigen/src/Core/util/Macros.h" _eigen3_version_header)
+  # Try the Eigen 5.x location first
+  if(EXISTS "${EIGEN3_INCLUDE_DIR}/Eigen/Version")
+    file(READ "${EIGEN3_INCLUDE_DIR}/Eigen/Version" _eigen3_version_header)
+  # Fall back to Eigen 3.x location
+  elseif(EXISTS "${EIGEN3_INCLUDE_DIR}/Eigen/src/Core/util/Macros.h")
+    file(READ "${EIGEN3_INCLUDE_DIR}/Eigen/src/Core/util/Macros.h" _eigen3_version_header)
+  else()
+    # Could not locate any known Eigen version header; mark version as not OK and return.
+    set(EIGEN3_VERSION_OK FALSE)
+    message(STATUS "Could not find Eigen version header under ${EIGEN3_INCLUDE_DIR}")
+    return()
+  endif()
 
   string(REGEX MATCH "define[ \t]+EIGEN_WORLD_VERSION[ \t]+([0-9]+)" _eigen3_world_version_match "${_eigen3_version_header}")
   set(EIGEN3_WORLD_VERSION "${CMAKE_MATCH_1}")