Brew error: Could not symlink, path is not writable

When I try to link a library with homebrew ( brew link libtool), I got the following error:

Linking /usr/local/Cellar/libtool/2.4.6… 
Error: Could not symlink lib/libltdl.7.dylib 
/usr/local/lib is not writable.

As explained here by Rick:

Start with brew doctor which will show you errors with your brew setup.

You might see something like this: "Warning: /usr/local/lib isn't writable."

It will give you the advice that: "You should probably chown /usr/local/lib/pkgconfig".

This means:  sudo chown {your-user-name} /usr/local/lib

Then you will need to link the files with this: brew link yourLibrary

If this does not work hopefully the output of brew doctor will give you enough to continue the search.