在按照WRF迁移指南移植过程中，需要添加external::derf这句话。如果不添加就会报如下错误：

clang-12: error: linker command failed with exit code 1 (use -v to see invocation)

Command exited with non-zero status 1

0.25user 0.03system 0:00.29elapsed 99%CPU (0avgtext+0avgdata 61632maxresident)k

0inputs+62080outputs (0major+1793minor)pagefaults 0swaps

make[1]: [Makefile:35: em_real] Error 1 (ignored)

请问报错原因是什么？这句话的作用又是什么？