make FFT a selective option

endif()

if(ENABLE_KSPACE)

  foreach(FFT FFTW3 MKL FFTW2)

    find_package(${FFT})

    if(${FFT}_FOUND)

  set(FFT "KISSFFT" CACHE STRING "FFT library for KSPACE package")

  set_property(CACHE FFT PROPERTY STRINGS KISSFFT FFTW3 MKL FFTW2)

  if(NOT FFT STREQUAL "KISSFFT")

    find_package(${FFT} REQUIRED)

    add_definitions(-DFFT_${FFT})

    include_directories(${${FFT}_INCLUDE_DIRS})

    list(APPEND LAMMPS_LINK_LIBS ${${FFT}_LIBRARIES})

      break()

  endif()

  endforeach()

  set(PACK_OPTIMIZATION "PACK_ARRAY" CACHE STRING "Optimization for FFT")

  set_property(CACHE LAMMPS_SIZE_LIMIT PROPERTY STRINGS PACK_ARRAY PACK_POINTER PACK_MEMCPY)

  if(NOT PACK_OPTIMIZATION STREQUAL "PACK_ARRAY")