USER-SMD.cmake: use Eigen3 imported target

cmake/Modules/Packages/USER-SMD.cmake

@@ -15,7 +15,7 @@ if(PKG_USER-SMD)
       CONFIGURE_COMMAND "" BUILD_COMMAND "" INSTALL_COMMAND ""
     )
     ExternalProject_get_property(Eigen3_build SOURCE_DIR)
-    set(EIGEN3_INCLUDE_DIR ${SOURCE_DIR})
+    target_include_directories(lammps ${SOURCE_DIR})
     add_dependencies(lammps Eigen3_build)
   else()
     find_package(Eigen3 NO_MODULE)
@@ -23,6 +23,6 @@ if(PKG_USER-SMD)
     if(NOT EIGEN3_FOUND)
       message(FATAL_ERROR "Eigen3 not found, help CMake to find it by setting EIGEN3_INCLUDE_DIR, or set DOWNLOAD_EIGEN3=ON to download it")
     endif()
+    target_link_libraries(lammps PRIVATE Eigen3::Eigen)
   endif()
-  include_directories(${EIGEN3_INCLUDE_DIR})
 endif()