我跑了：brew doctor
返回以下警告：
Warning: You have unlinked kegs in your Cellar
Leaving kegs unlinked can lead to build-trouble and cause brews that depend on those kegs to fail to run properly once built. Run `brew link` on these:
nettle
libtasn1
p11-kit
gnutls
我尝试运行brew link nettle，brew link --dry-run nettle和brew link --overwrite nettle
全部返回
`Linking /usr/local/Cellar/nettle/3.4... Error: File exists @ dir_s_mkdir - /usr/local/lib/pkgconfig`
请有人解释这意味着什么以及如何解决？