cmake: allow to build against an external libkokkos

if(PKG_KOKKOS)

  option(EXTERNAL_KOKKOS "Build against external kokkos library")

  if(EXTERNAL_KOKKOS)

    find_package(Kokkos REQUIRED)

    list(APPEND LAMMPS_LINK_LIBS Kokkos::kokkos)

  else()

    set(LAMMPS_LIB_KOKKOS_SRC_DIR ${LAMMPS_LIB_SOURCE_DIR}/kokkos)

    set(LAMMPS_LIB_KOKKOS_BIN_DIR ${LAMMPS_LIB_BINARY_DIR}/kokkos)

    add_definitions(-DLMP_KOKKOS)

                            ${LAMMPS_LIB_KOKKOS_BIN_DIR})

    include_directories(${Kokkos_INCLUDE_DIRS})

    list(APPEND LAMMPS_LINK_LIBS kokkos)

  endif()

  set(KOKKOS_PKG_SOURCES_DIR ${LAMMPS_SOURCE_DIR}/KOKKOS)

  set(KOKKOS_PKG_SOURCES ${KOKKOS_PKG_SOURCES_DIR}/kokkos.cpp