Brew doctor says: "Warning: /usr/local/include isn't writable."

Take ownership of it and everything in it.

Mac OS High Sierra or newer: (ty to Kirk in the comments below)

$ sudo chown -R $(whoami) $(brew --prefix)/*

Previous versions of macos:

$ sudo chown -R $USER:admin /usr/local/include

Then do another

$ brew doctor

What worked for me was too

sudo chmod g+w /usr/local
sudo chgrp staff /usr/local

What worked for me was

$ sudo chown -R yourname:admin /usr/local/bin