cmake: use NO_MODULE for Eigen3

endif()

if(PKG_USER-SMD)

  # hack alert!

  # something defines the Eigen3_DIR variable, but it has no effect

  # yet shows up in the TUI/GUI display as "not found".

  # and due to not being marked as advanced, it can mislead people

  # as the EIGEN3_INCLUDE_DIR variable which does have an effect

  # is marked as advanced. Tested on Fedora28 with Eigen3-3.3.5 rpm.

  mark_as_advanced(Eigen3_DIR)

  option(DOWNLOAD_EIGEN3 "Download Eigen3 (instead of using the system's one)" OFF)

  if(DOWNLOAD_EIGEN3)

    include(ExternalProject)

    set(EIGEN3_INCLUDE_DIR ${SOURCE_DIR})

    list(APPEND LAMMPS_DEPS Eigen3_build)

  else()

    find_package(Eigen3)

    find_package(Eigen3 NO_MODULE)

    mark_as_advanced(Eigen3_DIR)

    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()