cmake: don't build Eigen3 for USER-SMD

    ExternalProject_Add(Eigen3_build

      URL http://bitbucket.org/eigen/eigen/get/3.3.4.tar.gz 

      URL_MD5 1a47e78efe365a97de0c022d127607c3

      CMAKE_ARGS -DCMAKE_INSTALL_PREFIX=<INSTALL_DIR> -DEIGEN_TEST_NOQT=ON

        -DCMAKE_DISABLE_FIND_PACKAGE_LAPACK=ON -DCMAKE_DISABLE_FIND_PACKAGE_Cholmod=ON -DCMAKE_DISABLE_FIND_PACKAGE_Umfpack=ON -DCMAKE_DISABLE_FIND_PACKAGE_SuperLU=ON

        -DCMAKE_DISABLE_FIND_PACKAGE_PASTIX=ON -DCMAKE_DISABLE_FIND_PACKAGE_SPQR=ON -DCMAKE_DISABLE_FIND_PACKAGE_Boost=ON -DCMAKE_DISABLE_FIND_PACKAGE_CUDA=ON

        -DCMAKE_DISABLE_FIND_PACKAGE_FFTW=ON -DCMAKE_DISABLE_FIND_PACKAGE_MPFR=ON -DCMAKE_DISABLE_FIND_PACKAGE_OpenGL=ON

      CONFIGURE_COMMAND "" BUILD_COMMAND "" INSTALL_COMMAND ""

    )

    ExternalProject_get_property(Eigen3_build INSTALL_DIR)

    set(EIGEN3_INCLUDE_DIR ${INSTALL_DIR}/include/eigen3)

    ExternalProject_get_property(Eigen3_build SOURCE_DIR)

    set(EIGEN3_INCLUDE_DIR ${SOURCE_DIR})

    list(APPEND LAMMPS_DEPS Eigen3_build)

  else()

    find_package(Eigen3)