message: more precise empty directory pruning on install
diff: @@ -73,7 +73,7 @@ else ()
  \"\${MONGOCXX_INSTALL_MANIFEST_CONTENT}\")
  execute_process (
  COMMAND
  - find ${CMAKE_INSTALL_PREFIX} -type d -empty -delete
  + find \"\$ENV{DESTDIR}/${CMAKE_INSTALL_PREFIX}/${BSONCXX_HEADER_INSTALL_DIR}\" \"\$ENV{DESTDIR}/${CMAKE_INSTALL_PREFIX}/${MONGOCXX_HEADER_INSTALL_DIR}\" -type d -empty -delete
  )
  execute_process (
  COMMAND
  