Warning: Could not link pkg-config. Unlinking...
Error: The `brew link` step did not complete successfully
The formula built, but is not symlinked into /usr/local
Yo
As Michael Pigg said above, in my case it was the folder permission issue. I had to do the following:
sudo chmod 777 /usr/local/bin
sudo chmod 777 /usr/local/share/man/man1
brew link pkg-config
And after I'm done:
sudo chmod 755 /usr/local/bin
sudo chmod 755 /usr/local/share/man/man1
to restore the original permissions.
It may be a permissions issue on /usr/local/share. You need to have write permission in that directory for the link to complete. However, zsh will complain about the directory being insecure if you set the write permission. I gave the write permission to allow pkg-config to install and then removed it so that zsh would be happy.