tweak iwyu target. generate file with custom mappings

  if (IWYU_EXE AND IWYU_TOOL)

    add_custom_target(

      iwyu

      ${IWYU_TOOL} -o clang -p ${CMAKE_BINARY_DIR}

      WORKING_DIRECTORY ${CMAKE_BINARY_DIR}

      ${IWYU_TOOL} -o clang -p ${CMAKE_CURRENT_BINARY_DIR} -- -Xiwyu --mapping_file=${CMAKE_CURRENT_BINARY_DIR}/iwyu-extra-map.imp

      COMMENT "Running IWYU")

    add_dependencies(iwyu lammps)

    # generate file with additional mappings

    file(WRITE  ${CMAKE_CURRENT_BINARY_DIR}/iwyu-extra-map.imp "[\n")

    file(APPEND ${CMAKE_CURRENT_BINARY_DIR}/iwyu-extra-map.imp

           "  { include: [ \"<bits/types/struct_rusage.h>\", private, \"<sys/resource.h>\", public ] },\n")

    file(APPEND ${CMAKE_CURRENT_BINARY_DIR}/iwyu-extra-map.imp "]\n")

  else()

    message(FATAL_ERROR "To use IWYU you need the include-what-you-use/iwyu executable"

      "and the iwyu-tool/iwyu_tool script installed in your PATH")