external: don't link librt on apple.

external/InstallFlesnet.cmake

@@ -104,9 +104,8 @@ list(APPEND dir_to_link
      ${Boost_BEAST_LIBRARY}
     )
 if(NOT APPLE)
-  list(APPEND dir_to_link ${Boost_THREAD_LIBRARY})
+  list(APPEND dir_to_link ${Boost_THREAD_LIBRARY} -lrt)
 endif()
-list(APPEND dir_to_link -lrt)
 
 set_target_properties(external::fles_ipc PROPERTIES
   IMPORTED_LOCATION ${FLESNET_DESTDIR}/src/flesnet-build/lib/fles_ipc/${CMAKE_STATIC_LIBRARY_PREFIX}fles_ipc${CMAKE_STATIC_LIBRARY_SUFFIX}