cmake: make GPU_ARCH free form

      endif()

      option(CUDPP_OPT "Enable CUDPP_OPT" ON)

      set(GPU_ARCH "sm_30" CACHE STRING "LAMMPS GPU CUDA SM architecture")

      set_property(CACHE GPU_ARCH PROPERTY STRINGS sm_10 sm_20 sm_30 sm_60)

      set(GPU_ARCH "sm_30" CACHE STRING "LAMMPS GPU CUDA SM architecture (e.g.  sm_60)")

      file(GLOB GPU_LIB_CU ${LAMMPS_LIB_SOURCE_DIR}/gpu/*.cu ${CMAKE_CURRENT_SOURCE_DIR}/gpu/*.cu)

      list(REMOVE_ITEM GPU_LIB_CU ${LAMMPS_LIB_SOURCE_DIR}/gpu/lal_pppm.cu)