It doesn't really matter if the code as a whole worked without readline. What matters is if the code that linked to readline worked. It's that tiny bit of code that may or may not be a derived work. If it is, then you can't distribute it. No matter how tiny it is, if you can't distribute it, then you can't distribute it.

Keep in mind that agreeing to license your work under the GPL is an offer for remedying infringement. You don't have to accept. If you don't accept, then you have to deal with the consequences just like any other infringement case.

For the side not: It is true :-) However, just to be a bit playful, I should point out that fair use is not a defense in US copyright law. If your use is fair use it is still infringing. Fair use kicks in when determining damages. So your infringing use will suffer no damages because it is fair use. However, you should always be careful when relying on fair use because it is not a defense -- your infringement case can go to court and you still have to pay your lawyers!