small tweaks: improve messages and comments

foreach(_SRC ${SRC_AUTOGEN_FILES})

  get_filename_component(FILENAME "${_SRC}" NAME)

  if(EXISTS ${LAMMPS_SOURCE_DIR}/${FILENAME})

          message(FATAL_ERROR "\nFound file(s) generated by the make-based build system.\nPlease run 'make -C ${LAMMPS_SOURCE_DIR} purge'")

          message(FATAL_ERROR "\nFound header file(s) generated by the make-based build system.\nPlease run 'make -C ${LAMMPS_SOURCE_DIR} purge' to remove")

  endif()

endforeach()

# check for files installed by make-based buildsystem

# only run this time consuming check if there are new files

# check for files from packages installed by the make-based buildsystem

# this is slow, so only run this check if there are new files

if(NOT SRC_FILES STREQUAL SRC_FILES_CACHED)

  file(GLOB SRC_PKG_FILES ${LAMMPS_SOURCE_DIR}/*/*.cpp)

  message(STATUS "Running check for packages installed with 'make yes-<package>' (this may take a while)")

  foreach(_SRC ${SRC_PKG_FILES})

    get_filename_component(FILENAME "${_SRC}" NAME)

    if(EXISTS ${LAMMPS_SOURCE_DIR}/${FILENAME})

      message(FATAL_ERROR "\nFound package(s) installed by the make-based build system\nPlease run 'make -C ${LAMMPS_SOURCE_DIR} no-all purge'")

      message(FATAL_ERROR "\nFound package(s) installed by the make-based build system\nPlease run 'make -C ${LAMMPS_SOURCE_DIR} no-all purge' to uninstall")

    endif()

  endforeach()

  set(SRC_FILES_CACHED "${SRC_FILES}" CACHE INTERNAL "List of file in LAMMPS_SOURCE_DIR" FORCE)