message: fix extension build in gcc 7.x
diff: @@ -39,6 +39,8 @@ SET(ext_sources
  seafile_shell_ext.rc
  )
  +SET_SOURCE_FILES_PROPERTIES(context-menu.cpp PROPERTIES COMPILE_FLAGS -fpermissive)
  +
  INCLUDE_DIRECTORIES(${CMAKE_CURRENT_SOURCE_DIR})
  LINK_LIBRARIES(uuid oleaut32 ole32 ws2_32 shlwapi userenv)
  